On Tue, Nov 12, 2013 at 11:55:56AM +1100, NeilBrown wrote: > On Mon, 11 Nov 2013 22:47:57 +0800 [email protected] wrote: > > > 28cc2127527dcba2a0817afa8fd5a33c9e023090 is the first bad commit > > commit 28cc2127527dcba2a0817afa8fd5a33c9e023090 > > Author: Shaohua Li <[email protected]> > > Date: Tue Sep 10 15:37:56 2013 +0800 > > > > raid5: relieve lock contention in get_active_stripe() > > > > get_active_stripe() is the last place we have lock contention. It has > > two > > paths. One is stripe isn't found and new stripe is allocated, the other > > is > > stripe is found. > > > > The first path basically calls __find_stripe and init_stripe. It > > accesses > > conf->generation, conf->previous_raid_disks, conf->raid_disks, > > conf->prev_chunk_sectors, conf->chunk_sectors, conf->max_degraded, > > conf->prev_algo, conf->algorithm, the stripe_hashtbl and inactive_list. > > Except > > stripe_hashtbl and inactive_list, other fields are changed very rarely. > > > > With this patch, we split inactive_list and add new hash locks. Each > > free > > stripe belongs to a specific inactive list. Which inactive list is > > determined > > by stripe's lock_hash. Note, even a stripe hasn't a sector assigned, it > > has a > > lock_hash assigned. Stripe's inactive list is protected by a hash lock, > > which > > is determined by it's lock_hash too. The lock_hash is derivied from > > current > > stripe_hashtbl hash, which guarantees any stripe_hashtbl list will be > > assigned > > to a specific lock_hash, so we can use new hash lock to protect > > stripe_hashtbl > > list too. The goal of the new hash locks introduced is we can only use > > the new > > locks in the first path of get_active_stripe(). Since we have several > > hash > > locks, lock contention is relieved significantly. > > > > The first path of get_active_stripe() accesses other fields, since they > > are > > changed rarely, changing them now need take conf->device_lock and all > > hash > > locks. For a slow path, this isn't a problem. > > > > If we need lock device_lock and hash lock, we always lock hash lock > > first. The > > tricky part is release_stripe and friends. We need take device_lock > > first. > > Neil's suggestion is we put inactive stripes to a temporary list and > > readd it > > to inactive_list after device_lock is released. In this way, we add > > stripes to > > temporary list with device_lock hold and remove stripes from the list > > with hash > > lock hold. So we don't allow concurrent access to the temporary list, > > which > > means we need allocate temporary list for all participants of > > release_stripe. > > > > One downside is free stripes are maintained in their inactive list, > > they can't > > across between the lists. By default, we have total 256 stripes and 8 > > lists, so > > each list will have 32 stripes. It's possible one list has free stripe > > but > > other list hasn't. The chance should be rare because stripes allocation > > are > > even distributed. And we can always allocate more stripes for cache, > > several > > mega bytes memory isn't a big deal. > > > > This completely removes the lock contention of the first path of > > get_active_stripe(). It slows down the second code path a little bit > > though > > because we now need takes two locks, but since the hash lock isn't > > contended, > > the overhead should be quite small (several atomic instructions). The > > second > > path of get_active_stripe() (basically sequential write or big request > > size > > randwrite) still has lock contentions. > > > > Signed-off-by: Shaohua Li <[email protected]> > > Signed-off-by: NeilBrown <[email protected]> > > > > :040000 040000 84ab47136c389751c7c08ded47b1761b1bee7184 > > 351047cfe3ac66013fc5c77f23d9bb04f869081d M drivers > > bisect run success > > > > # bad: [86737931c2be292ec985df48f2e7fbafb4467f0e] Merge 'md/master' into > > devel-hourly-2013111107 > > # good: [5e01dc7b26d9f24f39abace5da98ccbd6a5ceb52] Linux 3.12 > > git bisect start '86737931c2be292ec985df48f2e7fbafb4467f0e' > > '5e01dc7b26d9f24f39abace5da98ccbd6a5ceb52' '--' > > # good: [21136946c495b0e1e0f7e25a8de6f170efbdeadf] drm/vmwgfx: fix warning > > if config intel iommu is off. > > git bisect good 21136946c495b0e1e0f7e25a8de6f170efbdeadf > > # good: [ee360d688c8e37f81c92039f76bebaddbe36befe] Merge branch > > 'acpi-assorted' > > git bisect good ee360d688c8e37f81c92039f76bebaddbe36befe > > # good: [cf0613d242805797f252535fcf7bb019512beb46] Merge branch > > 'gma500-next' of git://github.com/patjak/drm-gma500 into drm-next > > git bisect good cf0613d242805797f252535fcf7bb019512beb46 > > # good: [feba070dbac6f7b477570e590a7dc960b7b0f784] Merge branch 'pm-sleep' > > git bisect good feba070dbac6f7b477570e590a7dc960b7b0f784 > > # good: [6f092343855a71e03b8d209815d8c45bf3a27fcd] net: flow_dissector: > > fail on evil iph->ihl > > git bisect good 6f092343855a71e03b8d209815d8c45bf3a27fcd > > # good: [7d963128c95a790b40ae5bac6af23646ceffcb54] Merge 'drm/drm-next' > > into devel-hourly-2013111107 > > git bisect good 7d963128c95a790b40ae5bac6af23646ceffcb54 > > # bad: [917bb50339fbbea9c5f47d257ea42f9652129c3f] raid1: Replace > > raise_barrier/lower_barrier with freeze_array/unfreeze_array when > > reconfiguring the array. > > git bisect bad 917bb50339fbbea9c5f47d257ea42f9652129c3f > > # bad: [28cc2127527dcba2a0817afa8fd5a33c9e023090] raid5: relieve lock > > contention in get_active_stripe() > > git bisect bad 28cc2127527dcba2a0817afa8fd5a33c9e023090 > > # good: [09b06d70464536cb3cce7cf1c52f23a321604f62] md: fix calculation of > > stacking limits on level change. > > git bisect good 09b06d70464536cb3cce7cf1c52f23a321604f62 > > # good: [c0f773604d33616b07d61841463a056a780f5ae7] wait: add > > wait_event_cmd() > > git bisect good c0f773604d33616b07d61841463a056a780f5ae7 > > # first bad commit: [28cc2127527dcba2a0817afa8fd5a33c9e023090] raid5: > > relieve lock contention in get_active_stripe() > > > > I think I've fixed this by merging in the follow. > > Shaohua: could you please review and confirm if you agree? > > Thanks, > NeilBrown > > diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c > index c37ffca1b13c..93090b2afab4 100644 > --- a/drivers/md/raid5.c > +++ b/drivers/md/raid5.c > @@ -697,6 +697,7 @@ get_active_stripe(struct r5conf *conf, sector_t sector, > if (!test_bit(STRIPE_HANDLE, &sh->state)) > atomic_inc(&conf->active_stripes); > if (list_empty(&sh->lru) && > + !test_bit(STRIPE_ON_RELEASE_LIST, > &sh->state) && > !test_bit(STRIPE_EXPANDING, &sh->state)) > BUG(); > list_del_init(&sh->lru);
Yes, makes a lot of sense. Thanks! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
