Re: [dpdk-dev] [PATCH v4 1/3] lib/lpm: integrate RCU QSBR

[Ruifeng Wang <Ruifeng.Wang@arm.com>]

Hi Vladimir,

> -----Original Message-----
> From: Medvedkin, Vladimir <vladimir.medvedkin@intel.com>
> Sent: Friday, June 19, 2020 1:22 AM
> To: Ruifeng Wang <Ruifeng.Wang@arm.com>; Bruce Richardson
> <bruce.richardson@intel.com>; John McNamara
> <john.mcnamara@intel.com>; Marko Kovacevic
> <marko.kovacevic@intel.com>; Ray Kinsella <mdr@ashroe.eu>; Neil Horman
> <nhorman@tuxdriver.com>
> Cc: dev@dpdk.org; konstantin.ananyev@intel.com; Honnappa Nagarahalli
> <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v4 1/3] lib/lpm: integrate RCU QSBR
> 
> Hi Ruifeng,
> 
> Thanks for patches, see comments below
Thanks for your review.
> 
> 
> On 08/06/2020 06:16, Ruifeng Wang wrote:
> > Currently, the tbl8 group is freed even though the readers might be
> > using the tbl8 group entries. The freed tbl8 group can be reallocated
> > quickly. This results in incorrect lookup results.
> >
> > RCU QSBR process is integrated for safe tbl8 group reclaim.
> > Refer to RCU documentation to understand various aspects of
> > integrating RCU library into other libraries.
> >
> > Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> > ---
> >   doc/guides/prog_guide/lpm_lib.rst  |  32 ++++++++
> >   lib/librte_lpm/Makefile            |   2 +-
> >   lib/librte_lpm/meson.build         |   1 +
> >   lib/librte_lpm/rte_lpm.c           | 123 ++++++++++++++++++++++++++---
> >   lib/librte_lpm/rte_lpm.h           |  59 ++++++++++++++
> >   lib/librte_lpm/rte_lpm_version.map |   6 ++
> >   6 files changed, 211 insertions(+), 12 deletions(-)
> >
> > diff --git a/doc/guides/prog_guide/lpm_lib.rst
> > b/doc/guides/prog_guide/lpm_lib.rst
> > index 1609a57d0..7cc99044a 100644
> > --- a/doc/guides/prog_guide/lpm_lib.rst
> > +++ b/doc/guides/prog_guide/lpm_lib.rst
> > @@ -145,6 +145,38 @@ depending on whether we need to move to the
> next table or not.
> >   Prefix expansion is one of the keys of this algorithm,
> >   since it improves the speed dramatically by adding redundancy.
> >
> > +Deletion
> > +~~~~~~~~
> > +
> > +When deleting a rule, a replacement rule is searched for. Replacement
> > +rule is an existing rule that has the longest prefix match with the rule to be
> deleted, but has smaller depth.
> > +
> > +If a replacement rule is found, target tbl24 and tbl8 entries are
> > +updated to have the same depth and next hop value with the
> replacement rule.
> > +
> > +If no replacement rule can be found, target tbl24 and tbl8 entries will be
> cleared.
> > +
> > +Prefix expansion is performed if the rule's depth is not exactly 24 bits or
> 32 bits.
> > +
> > +After deleting a rule, a group of tbl8s that belongs to the same tbl24 entry
> are freed in following cases:
> > +
> > +*   All tbl8s in the group are empty .
> > +
> > +*   All tbl8s in the group have the same values and with depth no greater
> than 24.
> > +
> > +Free of tbl8s have different behaviors:
> > +
> > +*   If RCU is not used, tbl8s are cleared and reclaimed immediately.
> > +
> > +*   If RCU is used, tbl8s are reclaimed when readers are in quiescent state.
> > +
> > +When the LPM is not using RCU, tbl8 group can be freed immediately
> > +even though the readers might be using the tbl8 group entries. This might
> result in incorrect lookup results.
> > +
> > +RCU QSBR process is integrated for safe tbl8 group reclaimation.
> > +Application has certain responsibilities while using this feature.
> > +Please refer to resource reclaimation framework of :ref:`RCU library
> <RCU_Library>` for more details.
> > +
> >   Lookup
> >   ~~~~~~
> >
> > diff --git a/lib/librte_lpm/Makefile b/lib/librte_lpm/Makefile index
> > d682785b6..6f06c5c03 100644
> > --- a/lib/librte_lpm/Makefile
> > +++ b/lib/librte_lpm/Makefile
> > @@ -8,7 +8,7 @@ LIB = librte_lpm.a
> >
> >   CFLAGS += -O3
> >   CFLAGS += $(WERROR_FLAGS) -I$(SRCDIR) -LDLIBS += -lrte_eal
> > -lrte_hash
> > +LDLIBS += -lrte_eal -lrte_hash -lrte_rcu
> >
> >   EXPORT_MAP := rte_lpm_version.map
> >
> > diff --git a/lib/librte_lpm/meson.build b/lib/librte_lpm/meson.build
> > index 021ac6d8d..6cfc083c5 100644
> > --- a/lib/librte_lpm/meson.build
> > +++ b/lib/librte_lpm/meson.build
> > @@ -7,3 +7,4 @@ headers = files('rte_lpm.h', 'rte_lpm6.h')
> >   # without worrying about which architecture we actually need
> >   headers += files('rte_lpm_altivec.h', 'rte_lpm_neon.h', 'rte_lpm_sse.h')
> >   deps += ['hash']
> > +deps += ['rcu']
> > diff --git a/lib/librte_lpm/rte_lpm.c b/lib/librte_lpm/rte_lpm.c index
> > 38ab512a4..30f541179 100644
> > --- a/lib/librte_lpm/rte_lpm.c
> > +++ b/lib/librte_lpm/rte_lpm.c
> > @@ -1,5 +1,6 @@
> >   /* SPDX-License-Identifier: BSD-3-Clause
> >    * Copyright(c) 2010-2014 Intel Corporation
> > + * Copyright(c) 2020 Arm Limited
> >    */
> >
> >   #include <string.h>
> > @@ -246,12 +247,85 @@ rte_lpm_free(struct rte_lpm *lpm)
> >
> >   	rte_mcfg_tailq_write_unlock();
> >
> > +	if (lpm->dq)
> > +		rte_rcu_qsbr_dq_delete(lpm->dq);
> >   	rte_free(lpm->tbl8);
> >   	rte_free(lpm->rules_tbl);
> >   	rte_free(lpm);
> >   	rte_free(te);
> >   }
> >
> > +static void
> > +__lpm_rcu_qsbr_free_resource(void *p, void *data, unsigned int n) {
> > +	struct rte_lpm_tbl_entry zero_tbl8_entry = {0};
> > +	uint32_t tbl8_group_index = *(uint32_t *)data;
> > +	struct rte_lpm_tbl_entry *tbl8 = (struct rte_lpm_tbl_entry *)p;
> > +
> > +	RTE_SET_USED(n);
> > +	/* Set tbl8 group invalid */
> > +	__atomic_store(&tbl8[tbl8_group_index], &zero_tbl8_entry,
> > +		__ATOMIC_RELAXED);
> > +}
> > +
> > +/* Associate QSBR variable with an LPM object.
> > + */
> > +int
> > +rte_lpm_rcu_qsbr_add(struct rte_lpm *lpm, struct rte_lpm_rcu_config
> *cfg,
> > +	struct rte_rcu_qsbr_dq **dq)
> > +{
> > +	char rcu_dq_name[RTE_RCU_QSBR_DQ_NAMESIZE];
> > +	struct rte_rcu_qsbr_dq_parameters params = {0};
> > +
> > +	if ((lpm == NULL) || (cfg == NULL)) {
> > +		rte_errno = EINVAL;
> > +		return 1;
> > +	}
> > +
> > +	if (lpm->v) {
> > +		rte_errno = EEXIST;
> > +		return 1;
> > +	}
> > +
> > +	if (cfg->mode == RTE_LPM_QSBR_MODE_SYNC) {
> > +		/* No other things to do. */
> > +	} else if (cfg->mode == RTE_LPM_QSBR_MODE_DQ) {
> > +		/* Init QSBR defer queue. */
> > +		snprintf(rcu_dq_name, sizeof(rcu_dq_name),
> > +				"LPM_RCU_%s", lpm->name);
> > +		params.name = rcu_dq_name;
> > +		params.size = cfg->dq_size;
> > +		if (params.size == 0)
> > +			params.size = lpm->number_tbl8s;
> > +		params.trigger_reclaim_limit = cfg->reclaim_thd;
> > +		if (params.trigger_reclaim_limit == 0)
> 
> 
> This makes it impossible for a user to configure reclamation triggering with
> every call. Should we allow it?
Yes, use (reclaim_thd = 0) to trigger reclamation at each dq enqueue should be a valid case.
Will remove value overriding and take it as is.
> 
> 
> > +			params.trigger_reclaim_limit =
> > +					RTE_LPM_RCU_DQ_RECLAIM_THD;
> > +		params.max_reclaim_size = cfg->reclaim_max;
> > +		if (params.max_reclaim_size == 0)
> > +			params.max_reclaim_size =
> RTE_LPM_RCU_DQ_RECLAIM_MAX;
> > +		params.esize = sizeof(uint32_t);	/* tbl8 group index */
> > +		params.free_fn = __lpm_rcu_qsbr_free_resource;
> > +		params.p = lpm->tbl8;
> 
> 
> I think it's better to pass the LPM pointer here rather than tbl8, for
> example, in case we decide to add some counters in the future
Use LPM pointer is more extendable.
Will change in next version.
> 
> 
> > +		params.v = cfg->v;
> > +		lpm->dq = rte_rcu_qsbr_dq_create(&params);
> > +		if (lpm->dq == NULL) {
> > +			RTE_LOG(ERR, LPM,
> > +					"LPM QS defer queue creation
> failed\n");
> > +			return 1;
> > +		}
> > +		if (dq)
> > +			*dq = lpm->dq;
> > +	} else {
> > +		rte_errno = EINVAL;
> > +		return 1;
> > +	}
> > +	lpm->rcu_mode = cfg->mode;
> > +	lpm->v = cfg->v;
> > +
> > +	return 0;
> > +}
> > +
> >   /*
> >    * Adds a rule to the rule table.
> >    *
> > @@ -394,14 +468,15 @@ rule_find(struct rte_lpm *lpm, uint32_t
> ip_masked, uint8_t depth)
> >    * Find, clean and allocate a tbl8.
> >    */
> >   static int32_t
> > -tbl8_alloc(struct rte_lpm_tbl_entry *tbl8, uint32_t number_tbl8s)
> > +_tbl8_alloc(struct rte_lpm *lpm)
> >   {
> >   	uint32_t group_idx; /* tbl8 group index. */
> >   	struct rte_lpm_tbl_entry *tbl8_entry;
> >
> >   	/* Scan through tbl8 to find a free (i.e. INVALID) tbl8 group. */
> > -	for (group_idx = 0; group_idx < number_tbl8s; group_idx++) {
> > -		tbl8_entry = &tbl8[group_idx *
> RTE_LPM_TBL8_GROUP_NUM_ENTRIES];
> > +	for (group_idx = 0; group_idx < lpm->number_tbl8s; group_idx++) {
> > +		tbl8_entry = &lpm->tbl8[group_idx *
> > +
> 	RTE_LPM_TBL8_GROUP_NUM_ENTRIES];
> >   		/* If a free tbl8 group is found clean it and set as VALID. */
> >   		if (!tbl8_entry->valid_group) {
> >   			struct rte_lpm_tbl_entry new_tbl8_entry = {
> > @@ -427,14 +502,40 @@ tbl8_alloc(struct rte_lpm_tbl_entry *tbl8,
> uint32_t number_tbl8s)
> >   	return -ENOSPC;
> >   }
> >
> > +static int32_t
> > +tbl8_alloc(struct rte_lpm *lpm)
> > +{
> > +	int32_t group_idx; /* tbl8 group index. */
> > +
> > +	group_idx = _tbl8_alloc(lpm);
> > +	if ((group_idx < 0) && (lpm->dq != NULL)) {
> 
> 
> I think (group_idx == -ENOSPC) will be safer
Will change it in next version.
> 
> 
> > +		/* If there are no tbl8 groups try to reclaim one. */
> > +		if (rte_rcu_qsbr_dq_reclaim(lpm->dq, 1, NULL, NULL, NULL)
> == 0)
> > +			group_idx = _tbl8_alloc(lpm);
> 
> 
> I'm not really happy with this approach. _tbl8_alloc() produces linear
> scan through the memory to find a free group_idx and it is the slowest
> part of rte_lpm_add().
> Here after reclamation of some group index we need to rescan a memory
> again to find it. It would be great if there will be some way to return
> reclaimed elements. Or just to dequeue elements from dq and reclaim them
> manually.
I think there is little chance a rescan will be needed. If RCU QSBR defer queue trigger_reclaim_limit
is configured with reasonable value, tbl8 groups will be reclaimed regularly. So defer queue won't get too long.

Return reclaimed elements makes API complex. Not sure if it is useful for other use cases.
@Honnappa Nagarahalli, any idea?
> 
> 
> > +	}
> > +
> > +	return group_idx;
> > +}
> > +
> >   static void
> > -tbl8_free(struct rte_lpm_tbl_entry *tbl8, uint32_t tbl8_group_start)
> > +tbl8_free(struct rte_lpm *lpm, uint32_t tbl8_group_start)
> >   {
> > -	/* Set tbl8 group invalid*/
> >   	struct rte_lpm_tbl_entry zero_tbl8_entry = {0};
> >
> > -	__atomic_store(&tbl8[tbl8_group_start], &zero_tbl8_entry,
> > -			__ATOMIC_RELAXED);
> > +	if (!lpm->v) {
> > +		/* Set tbl8 group invalid*/
> > +		__atomic_store(&lpm->tbl8[tbl8_group_start],
> &zero_tbl8_entry,
> > +				__ATOMIC_RELAXED);
> > +	} else if (lpm->rcu_mode == RTE_LPM_QSBR_MODE_SYNC) {
> > +		/* Wait for quiescent state change. */
> > +		rte_rcu_qsbr_synchronize(lpm->v,
> RTE_QSBR_THRID_INVALID);
> > +		/* Set tbl8 group invalid*/
> > +		__atomic_store(&lpm->tbl8[tbl8_group_start],
> &zero_tbl8_entry,
> > +				__ATOMIC_RELAXED);
> > +	} else if (lpm->rcu_mode == RTE_LPM_QSBR_MODE_DQ) {
> > +		/* Push into QSBR defer queue. */
> > +		rte_rcu_qsbr_dq_enqueue(lpm->dq, (void
> *)&tbl8_group_start);
> > +	}
> >   }
> >
> >   static __rte_noinline int32_t
> > @@ -523,7 +624,7 @@ add_depth_big(struct rte_lpm *lpm, uint32_t
> ip_masked, uint8_t depth,
> >
> >   	if (!lpm->tbl24[tbl24_index].valid) {
> >   		/* Search for a free tbl8 group. */
> > -		tbl8_group_index = tbl8_alloc(lpm->tbl8, lpm-
> >number_tbl8s);
> > +		tbl8_group_index = tbl8_alloc(lpm);
> >
> >   		/* Check tbl8 allocation was successful. */
> >   		if (tbl8_group_index < 0) {
> > @@ -569,7 +670,7 @@ add_depth_big(struct rte_lpm *lpm, uint32_t
> ip_masked, uint8_t depth,
> >   	} /* If valid entry but not extended calculate the index into Table8. */
> >   	else if (lpm->tbl24[tbl24_index].valid_group == 0) {
> >   		/* Search for free tbl8 group. */
> > -		tbl8_group_index = tbl8_alloc(lpm->tbl8, lpm-
> >number_tbl8s);
> > +		tbl8_group_index = tbl8_alloc(lpm);
> >
> >   		if (tbl8_group_index < 0) {
> >   			return tbl8_group_index;
> > @@ -977,7 +1078,7 @@ delete_depth_big(struct rte_lpm *lpm, uint32_t
> ip_masked,
> >   		 */
> >   		lpm->tbl24[tbl24_index].valid = 0;
> >   		__atomic_thread_fence(__ATOMIC_RELEASE);
> > -		tbl8_free(lpm->tbl8, tbl8_group_start);
> > +		tbl8_free(lpm, tbl8_group_start);
> >   	} else if (tbl8_recycle_index > -1) {
> >   		/* Update tbl24 entry. */
> >   		struct rte_lpm_tbl_entry new_tbl24_entry = {
> > @@ -993,7 +1094,7 @@ delete_depth_big(struct rte_lpm *lpm, uint32_t
> ip_masked,
> >   		__atomic_store(&lpm->tbl24[tbl24_index],
> &new_tbl24_entry,
> >   				__ATOMIC_RELAXED);
> >   		__atomic_thread_fence(__ATOMIC_RELEASE);
> > -		tbl8_free(lpm->tbl8, tbl8_group_start);
> > +		tbl8_free(lpm, tbl8_group_start);
> >   	}
> >   #undef group_idx
> >   	return 0;
> > diff --git a/lib/librte_lpm/rte_lpm.h b/lib/librte_lpm/rte_lpm.h
> > index b9d49ac87..8c054509a 100644
> > --- a/lib/librte_lpm/rte_lpm.h
> > +++ b/lib/librte_lpm/rte_lpm.h
> > @@ -1,5 +1,6 @@
> >   /* SPDX-License-Identifier: BSD-3-Clause
> >    * Copyright(c) 2010-2014 Intel Corporation
> > + * Copyright(c) 2020 Arm Limited
> >    */
> >
> >   #ifndef _RTE_LPM_H_
> > @@ -20,6 +21,7 @@
> >   #include <rte_memory.h>
> >   #include <rte_common.h>
> >   #include <rte_vect.h>
> > +#include <rte_rcu_qsbr.h>
> >
> >   #ifdef __cplusplus
> >   extern "C" {
> > @@ -62,6 +64,17 @@ extern "C" {
> >   /** Bitmask used to indicate successful lookup */
> >   #define RTE_LPM_LOOKUP_SUCCESS          0x01000000
> >
> > +/** @internal Default threshold to trigger RCU defer queue reclaimation.
> */
> > +#define RTE_LPM_RCU_DQ_RECLAIM_THD	32
> > +
> > +/** @internal Default RCU defer queue entries to reclaim in one go. */
> > +#define RTE_LPM_RCU_DQ_RECLAIM_MAX	16
> > +
> > +/* Create defer queue for reclaim. */
> > +#define RTE_LPM_QSBR_MODE_DQ		0
> > +/* Use blocking mode reclaim. No defer queue created. */
> > +#define RTE_LPM_QSBR_MODE_SYNC		0x01
> 
> 
> using enums instead of defines?
Will convert to enums in next version.
> 
> 
> > +
> >   #if RTE_BYTE_ORDER == RTE_LITTLE_ENDIAN
> >   /** @internal Tbl24 entry structure. */
> >   __extension__
> > @@ -130,6 +143,28 @@ struct rte_lpm {
> >   			__rte_cache_aligned; /**< LPM tbl24 table. */
> >   	struct rte_lpm_tbl_entry *tbl8; /**< LPM tbl8 table. */
> >   	struct rte_lpm_rule *rules_tbl; /**< LPM rules. */
> > +
> > +	/* RCU config. */
> > +	struct rte_rcu_qsbr *v;		/* RCU QSBR variable. */
> > +	uint32_t rcu_mode;		/* Blocking, defer queue. */
> > +	struct rte_rcu_qsbr_dq *dq;	/* RCU QSBR defer queue. */
> > +};
> > +
> > +/** LPM RCU QSBR configuration structure. */
> > +struct rte_lpm_rcu_config {
> > +	struct rte_rcu_qsbr *v;	/* RCU QSBR variable. */
> > +	/* Mode of RCU QSBR. RTE_LPM_QSBR_MODE_xxx
> > +	 * '0' for default: create defer queue for reclaim.
> > +	 */
> > +	uint32_t mode;
> > +	/* RCU defer queue size. default: lpm->number_tbl8s. */
> > +	uint32_t dq_size;
> > +	uint32_t reclaim_thd;	/* Threshold to trigger auto reclaim.
> > +				 * default:
> RTE_LPM_RCU_DQ_RECLAIM_TRHD.
> > +				 */
> > +	uint32_t reclaim_max;	/* Max entries to reclaim in one go.
> > +				 * default:
> RTE_LPM_RCU_DQ_RECLAIM_MAX.
> > +				 */
> >   };
> >
> >   /**
> > @@ -179,6 +214,30 @@ rte_lpm_find_existing(const char *name);
> >   void
> >   rte_lpm_free(struct rte_lpm *lpm);
> >
> > +/**
> > + * @warning
> > + * @b EXPERIMENTAL: this API may change without prior notice
> > + *
> > + * Associate RCU QSBR variable with an LPM object.
> > + *
> > + * @param lpm
> > + *   the lpm object to add RCU QSBR
> > + * @param cfg
> > + *   RCU QSBR configuration
> > + * @param dq
> > + *   handler of created RCU QSBR defer queue
> > + * @return
> > + *   On success - 0
> > + *   On error - 1 with error code set in rte_errno.
> > + *   Possible rte_errno codes are:
> > + *   - EINVAL - invalid pointer
> > + *   - EEXIST - already added QSBR
> > + *   - ENOMEM - memory allocation failure
> > + */
> > +__rte_experimental
> > +int rte_lpm_rcu_qsbr_add(struct rte_lpm *lpm, struct
> rte_lpm_rcu_config *cfg,
> > +	struct rte_rcu_qsbr_dq **dq);
> > +
> >   /**
> >    * Add a rule to the LPM table.
> >    *
> > diff --git a/lib/librte_lpm/rte_lpm_version.map
> b/lib/librte_lpm/rte_lpm_version.map
> > index 500f58b80..bfccd7eac 100644
> > --- a/lib/librte_lpm/rte_lpm_version.map
> > +++ b/lib/librte_lpm/rte_lpm_version.map
> > @@ -21,3 +21,9 @@ DPDK_20.0 {
> >
> >   	local: *;
> >   };
> > +
> > +EXPERIMENTAL {
> > +	global:
> > +
> > +	rte_lpm_rcu_qsbr_add;
> > +};
> 
> --
> Regards,
> Vladimir