From: "Mattias Rönnblom" <hofors@lysator.liu.se>
To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
"dev@dpdk.org" <dev@dpdk.org>
Cc: "thomas@monjalon.net" <thomas@monjalon.net>,
"David Marchand" <david.marchand@redhat.com>,
"onar.olsen@ericsson.com" <onar.olsen@ericsson.com>,
nd <nd@arm.com>,
"konstantin.ananyev@intel.com" <konstantin.ananyev@intel.com>,
"mb@smartsharesystems.com" <mb@smartsharesystems.com>,
"stephen@networkplumber.org" <stephen@networkplumber.org>,
"Ola Liljedahl" <Ola.Liljedahl@arm.com>,
"Mattias Rönnblom" <mattias.ronnblom@ericsson.com>
Subject: Re: [PATCH v3] eal: add seqlock
Date: Fri, 8 Apr 2022 15:45:32 +0200 [thread overview]
Message-ID: <d1cb1668-6879-c353-beea-78e8c61e1fe7@lysator.liu.se> (raw)
In-Reply-To: <DBAPR08MB5814774A834186EE7272BE5C98E29@DBAPR08MB5814.eurprd08.prod.outlook.com>
On 2022-04-03 19:37, Honnappa Nagarahalli wrote:
> <snip>
>
>>
>>>> + * Example usage:
>>>> + * @code{.c}
>>>> + * #define MAX_Y_LEN (16)
>>>> + * // Application-defined example data structure, protected by a seqlock.
>>>> + * struct config {
>>>> + * rte_seqlock_t lock;
>>>> + * int param_x;
>>>> + * char param_y[MAX_Y_LEN];
>>>> + * };
>>>> + *
>>>> + * // Accessor function for reading config fields.
>>>> + * void
>>>> + * config_read(const struct config *config, int *param_x, char
>>>> +*param_y)
>>>> + * {
>>>> + * // Temporary variables, just to improve readability.
>>> I think the above comment is not necessary. It is beneficial to copy the
>> protected data to keep the read side critical section small.
>>>
>>
>> The data here would be copied into the buffers supplied by config_read()
>> anyways, so it's a copy regardless.
> I see what you mean here. I would think the local variables add confusion, the copy can happen to the passed parameters directly. I will leave it to you to decide.
>
I'll remove the temp variables.
>>
>>>> + * int tentative_x;
>>>> + * char tentative_y[MAX_Y_LEN];
>>>> + * uint32_t sn;
>>>> + *
>>>> + * sn = rte_seqlock_read_lock(&config->lock);
>>>> + * do {
>>>> + * // Loads may be atomic or non-atomic, as in this example.
>>>> + * tentative_x = config->param_x;
>>>> + * strcpy(tentative_y, config->param_y);
>>>> + * } while (!rte_seqlock_read_tryunlock(&config->lock, &sn));
>>>> + * // An application could skip retrying, and try again later, if
>>>> + * // progress is possible without the data.
>>>> + *
>>>> + * *param_x = tentative_x;
>>>> + * strcpy(param_y, tentative_y);
>>>> + * }
>>>> + *
>>>> + * // Accessor function for writing config fields.
>>>> + * void
>>>> + * config_update(struct config *config, int param_x, const char
>>>> +*param_y)
>>>> + * {
>>>> + * rte_seqlock_write_lock(&config->lock);
>>>> + * // Stores may be atomic or non-atomic, as in this example.
>>>> + * config->param_x = param_x;
>>>> + * strcpy(config->param_y, param_y);
>>>> + * rte_seqlock_write_unlock(&config->lock);
>>>> + * }
>>>> + * @endcode
>>>> + *
>>>> + * @see
>>>> + * https://en.wikipedia.org/wiki/Seqlock.
>>>> + */
>>>> +
>>>> +#include <stdbool.h>
>>>> +#include <stdint.h>
>>>> +
>>>> +#include <rte_atomic.h>
>>>> +#include <rte_branch_prediction.h>
>>>> +#include <rte_spinlock.h>
>>>> +
>>>> +/**
>>>> + * The RTE seqlock type.
>>>> + */
>>>> +typedef struct {
>>>> + uint32_t sn; /**< A sequence number for the protected data. */
>>>> + rte_spinlock_t lock; /**< Spinlock used to serialize writers. */ }
>>> Suggest using ticket lock for the writer side. It should have low overhead
>> when there is a single writer, but provides better functionality when there are
>> multiple writers.
>>>
>>
>> Is a seqlock the synchronization primitive of choice for high-contention cases?
>> I would say no, but I'm not sure what you would use instead.
> I think Stephen has come across some use cases of high contention writers with readers, maybe Stephen can provide some input.
>
> IMO, there is no harm/perf issues in using ticket lock.
>
OK. I will leave at as spinlock for now (PATCH v4).
next prev parent reply other threads:[~2022-04-08 13:45 UTC|newest]
Thread overview: 104+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-22 16:10 DPDK seqlock Mattias Rönnblom
2022-03-22 16:46 ` Ananyev, Konstantin
2022-03-24 4:52 ` Honnappa Nagarahalli
2022-03-24 5:06 ` Stephen Hemminger
2022-03-24 11:34 ` Mattias Rönnblom
2022-03-25 20:24 ` [RFC] eal: add seqlock Mattias Rönnblom
2022-03-25 21:10 ` Stephen Hemminger
2022-03-26 14:57 ` Mattias Rönnblom
2022-03-27 14:49 ` Ananyev, Konstantin
2022-03-27 17:42 ` Mattias Rönnblom
2022-03-28 10:53 ` Ananyev, Konstantin
2022-03-28 14:06 ` Ola Liljedahl
2022-03-29 8:32 ` Mattias Rönnblom
2022-03-29 13:20 ` Ananyev, Konstantin
2022-03-30 10:07 ` [PATCH] " Mattias Rönnblom
2022-03-30 10:50 ` Morten Brørup
2022-03-30 11:24 ` Tyler Retzlaff
2022-03-30 11:25 ` Mattias Rönnblom
2022-03-30 14:26 ` [PATCH v2] " Mattias Rönnblom
2022-03-31 7:46 ` Mattias Rönnblom
2022-03-31 9:04 ` Ola Liljedahl
2022-03-31 9:25 ` Morten Brørup
2022-03-31 9:38 ` Ola Liljedahl
2022-03-31 10:03 ` Morten Brørup
2022-03-31 11:44 ` Ola Liljedahl
2022-03-31 11:50 ` Morten Brørup
2022-03-31 14:02 ` Mattias Rönnblom
2022-04-01 15:07 ` [PATCH v3] " Mattias Rönnblom
2022-04-02 0:21 ` Honnappa Nagarahalli
2022-04-02 11:01 ` Morten Brørup
2022-04-02 19:38 ` Honnappa Nagarahalli
2022-04-10 13:51 ` [RFC 1/3] eal: add macro to warn for unused function return values Mattias Rönnblom
2022-04-10 13:51 ` [RFC 2/3] eal: emit warning for unused trylock return value Mattias Rönnblom
2022-04-10 13:51 ` [RFC 3/3] examples/bond: fix invalid use of trylock Mattias Rönnblom
2022-04-11 1:01 ` Min Hu (Connor)
2022-04-11 14:32 ` Mattias Rönnblom
2022-04-11 11:25 ` David Marchand
2022-04-11 14:33 ` Mattias Rönnblom
2022-04-10 18:02 ` [RFC 1/3] eal: add macro to warn for unused function return values Stephen Hemminger
2022-04-10 18:50 ` Mattias Rönnblom
2022-04-11 7:17 ` Morten Brørup
2022-04-11 14:29 ` Mattias Rönnblom
2022-04-11 9:16 ` Bruce Richardson
2022-04-11 14:27 ` Mattias Rönnblom
2022-04-11 15:15 ` [PATCH " Mattias Rönnblom
2022-04-11 15:15 ` [PATCH 2/3] eal: emit warning for unused trylock return value Mattias Rönnblom
2022-04-11 15:29 ` Morten Brørup
2022-04-11 15:15 ` [PATCH 3/3] examples/bond: fix invalid use of trylock Mattias Rönnblom
2022-04-14 12:06 ` David Marchand
2022-04-11 15:25 ` [PATCH 1/3] eal: add macro to warn for unused function return values Morten Brørup
2022-04-11 18:24 ` [RFC " Tyler Retzlaff
2022-04-03 6:10 ` [PATCH v3] eal: add seqlock Mattias Rönnblom
2022-04-03 17:27 ` Honnappa Nagarahalli
2022-04-03 18:37 ` Ola Liljedahl
2022-04-04 21:56 ` Honnappa Nagarahalli
2022-04-03 6:33 ` Mattias Rönnblom
2022-04-03 17:37 ` Honnappa Nagarahalli
2022-04-08 13:45 ` Mattias Rönnblom [this message]
2022-04-02 18:15 ` Ola Liljedahl
2022-04-02 19:31 ` Honnappa Nagarahalli
2022-04-02 20:36 ` Morten Brørup
2022-04-02 22:01 ` Honnappa Nagarahalli
2022-04-03 18:11 ` Ola Liljedahl
2022-04-03 6:51 ` Mattias Rönnblom
2022-03-31 13:51 ` [PATCH v2] " Mattias Rönnblom
2022-04-02 0:54 ` Stephen Hemminger
2022-04-02 10:25 ` Morten Brørup
2022-04-02 17:43 ` Ola Liljedahl
2022-03-31 13:38 ` Mattias Rönnblom
2022-03-31 14:53 ` Ola Liljedahl
2022-04-02 0:52 ` Stephen Hemminger
2022-04-03 6:23 ` Mattias Rönnblom
2022-04-02 0:50 ` Stephen Hemminger
2022-04-02 17:54 ` Ola Liljedahl
2022-04-02 19:37 ` Honnappa Nagarahalli
2022-04-05 20:16 ` Stephen Hemminger
2022-04-08 13:50 ` Mattias Rönnblom
2022-04-08 14:24 ` [PATCH v4] " Mattias Rönnblom
2022-04-08 15:17 ` Stephen Hemminger
2022-04-08 16:24 ` Mattias Rönnblom
2022-04-08 15:19 ` Stephen Hemminger
2022-04-08 16:37 ` Mattias Rönnblom
2022-04-08 16:48 ` Mattias Rönnblom
2022-04-12 17:27 ` Ananyev, Konstantin
2022-04-28 10:28 ` David Marchand
2022-05-01 13:46 ` Mattias Rönnblom
2022-05-01 14:03 ` [PATCH v5] " Mattias Rönnblom
2022-05-01 14:22 ` Mattias Rönnblom
2022-05-02 6:47 ` David Marchand
2022-05-01 20:17 ` Stephen Hemminger
2022-05-02 4:51 ` Mattias Rönnblom
2022-05-06 1:26 ` fengchengwen
2022-05-06 1:33 ` Honnappa Nagarahalli
2022-05-06 4:17 ` fengchengwen
2022-05-06 5:19 ` Honnappa Nagarahalli
2022-05-06 7:03 ` fengchengwen
2022-05-08 11:56 ` Mattias Rönnblom
2022-05-08 12:12 ` [PATCH v6] " Mattias Rönnblom
2022-05-08 16:10 ` Stephen Hemminger
2022-05-08 19:40 ` Mattias Rönnblom
2022-05-09 3:48 ` Stephen Hemminger
2022-05-09 6:26 ` Morten Brørup
2022-05-13 6:27 ` Mattias Rönnblom
2022-03-23 12:04 ` DPDK seqlock Morten Brørup
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