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From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
To: "Wang, Yipeng1" <yipeng1.wang@intel.com>,
	Stephen Hemminger <stephen@networkplumber.org>
Cc: "Gobriel, Sameh" <sameh.gobriel@intel.com>,
	"Richardson, Bruce" <bruce.richardson@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	"De Lara Guarch,  Pablo" <pablo.de.lara.guarch@intel.com>,
	nd <nd@arm.com>,
	Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
	nd <nd@arm.com>
Subject: Re: [dpdk-dev] [PATCH] hash: document breakage with multi-writer thread
Date: Thu, 4 Jun 2020 19:34:19 +0000
Message-ID: <AM4PR0802MB2212599ACC592EC4A42965A298890@AM4PR0802MB2212.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <BYAPR11MB3494A1653D9DD9B4958F6BD7C3890@BYAPR11MB3494.namprd11.prod.outlook.com>

<snip>

> > > >
> > > > > Subject: [PATCH] hash: document breakage with multi-writer
> > > > > thread
> > > > >
> > > > > The code in rte_cuckoo_hash multi-writer support is broken if
> > > > > write operations are called from a non-EAL thread.
> > > > >
> > > > > rte_lcore_id() wil return LCORE_ID_ANY (UINT32_MAX) for non EAL
> > > > > thread and that leads to using wrong local cache.
> > > > >
> > > > > Add error checks and document the restriction.
> > > > Having multiple non-EAL writer threads is a valid use case. Should
> > > > we fix the
> > > issue instead?
> > >
> > > Discovered this the hard way...
> > >
> > > Fixing is non-trivial. Basically, the local cache has to be take out
> > > and that leads to having to do real locking or atomic operations.
> > Looking at rte_hash_create function:
> >
> >         if (params->extra_flag &
> > RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD) {
> >                 use_local_cache = 1;
> >                 writer_takes_lock = 1;
> >         }
> >
> > The writer locks are in place already. The code to handle the case
> > when local cache is taken out is also there.
> > What we need is another input flag that says 'multi writer + non-eal
> threads'
> > which would set 'use_local_cache = 0' and 'writer_takes_lock = 1'.
> > Not sure, it would be valuable addition. But looks like this is what
> > you were expecting when you had enabled
> > 'RTE_HASH_EXTRA_FLAGS_MULTI_WRITER_ADD'. Many other APIs in DPDK
> do
> > not provide this kind of MT safety.
> 
> [Wang, Yipeng]
> If possible, we can try to not add new flags, because there are already a lot of
> flag options.
> How about in the code, we check if the writer is a non-eal or not by checking
> the rte_lcore_id, and operate on the global queue?
> Could this work?
> If(h->use_local_cache) {
> 	lcore_id = rte_lcore_id();
> 	if(lcore_id == LCORE_ID_ANY) {   // this is non-eal threads
> 		<call rte_ring_mp/mc_* to directly operate on global queue>
> 	}
> 	Else {
> 		<original path>
> 	}
> }
The other thing I wanted to do was saving on the memory allocated for the local cache when the writers are non-eal threads. Without knowing the kind of threads upfront, we might have to create the local cache when a writer adds the entry first time.

  reply	other threads:[~2020-06-04 19:34 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-04 17:17 Stephen Hemminger
2020-06-04 17:51 ` Honnappa Nagarahalli
2020-06-04 17:58   ` Stephen Hemminger
2020-06-04 18:43     ` Honnappa Nagarahalli
2020-06-04 19:10       ` Wang, Yipeng1
2020-06-04 19:34         ` Honnappa Nagarahalli [this message]
2020-06-04 20:22           ` Wang, Yipeng1
2020-06-04 21:06             ` Stephen Hemminger
2020-06-04 21:32 ` Wang, Yipeng1
2020-06-05 18:35 ` Stephen Hemminger
2020-06-16 16:12   ` Thomas Monjalon
2020-11-26 17:56     ` Thomas Monjalon

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