Messages in this thread | | | Date | Fri, 24 Feb 2023 18:12:31 +0800 | Subject | Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless | From | Qi Zheng <> |
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On 2023/2/24 16:20, Sultan Alsawaf wrote: > On Fri, Feb 24, 2023 at 12:00:21PM +0800, Qi Zheng wrote: >> >> >> On 2023/2/24 02:24, Sultan Alsawaf wrote: >>> On Thu, Feb 23, 2023 at 09:27:20PM +0800, Qi Zheng wrote: >>>> The shrinker_rwsem is a global lock in shrinkers subsystem, >>>> it is easy to cause blocking in the following cases: >>>> >>>> a. the write lock of shrinker_rwsem was held for too long. >>>> For example, there are many memcgs in the system, which >>>> causes some paths to hold locks and traverse it for too >>>> long. (e.g. expand_shrinker_info()) >>>> b. the read lock of shrinker_rwsem was held for too long, >>>> and a writer came at this time. Then this writer will be >>>> forced to wait and block all subsequent readers. >>>> For example: >>>> - be scheduled when the read lock of shrinker_rwsem is >>>> held in do_shrink_slab() >>>> - some shrinker are blocked for too long. Like the case >>>> mentioned in the patchset[1]. >>>> >>>> Therefore, many times in history ([2],[3],[4],[5]), some >>>> people wanted to replace shrinker_rwsem reader with SRCU, >>>> but they all gave up because SRCU was not unconditionally >>>> enabled. >>>> >>>> But now, since commit 1cd0bd06093c ("rcu: Remove CONFIG_SRCU"), >>>> the SRCU is unconditionally enabled. So it's time to use >>>> SRCU to protect readers who previously held shrinker_rwsem. >>>> >>>> [1]. https://lore.kernel.org/lkml/20191129214541.3110-1-ptikhomirov@virtuozzo.com/ >>>> [2]. https://lore.kernel.org/all/1437080113.3596.2.camel@stgolabs.net/ >>>> [3]. https://lore.kernel.org/lkml/1510609063-3327-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/ >>>> [4]. https://lore.kernel.org/lkml/153365347929.19074.12509495712735843805.stgit@localhost.localdomain/ >>>> [5]. https://lore.kernel.org/lkml/20210927074823.5825-1-sultan@kerneltoast.com/ >>>> >>>> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com> >>>> --- >>>> mm/vmscan.c | 27 +++++++++++---------------- >>>> 1 file changed, 11 insertions(+), 16 deletions(-) >>>> >>>> diff --git a/mm/vmscan.c b/mm/vmscan.c >>>> index 9f895ca6216c..02987a6f95d1 100644 >>>> --- a/mm/vmscan.c >>>> +++ b/mm/vmscan.c >>>> @@ -202,6 +202,7 @@ static void set_task_reclaim_state(struct task_struct *task, >>>> LIST_HEAD(shrinker_list); >>>> DECLARE_RWSEM(shrinker_rwsem); >>>> +DEFINE_SRCU(shrinker_srcu); >>>> #ifdef CONFIG_MEMCG >>>> static int shrinker_nr_max; >>>> @@ -706,7 +707,7 @@ void free_prealloced_shrinker(struct shrinker *shrinker) >>>> void register_shrinker_prepared(struct shrinker *shrinker) >>>> { >>>> down_write(&shrinker_rwsem); >>>> - list_add_tail(&shrinker->list, &shrinker_list); >>>> + list_add_tail_rcu(&shrinker->list, &shrinker_list); >>>> shrinker->flags |= SHRINKER_REGISTERED; >>>> shrinker_debugfs_add(shrinker); >>>> up_write(&shrinker_rwsem); >>>> @@ -760,13 +761,15 @@ void unregister_shrinker(struct shrinker *shrinker) >>>> return; >>>> down_write(&shrinker_rwsem); >>>> - list_del(&shrinker->list); >>>> + list_del_rcu(&shrinker->list); >>>> shrinker->flags &= ~SHRINKER_REGISTERED; >>>> if (shrinker->flags & SHRINKER_MEMCG_AWARE) >>>> unregister_memcg_shrinker(shrinker); >>>> debugfs_entry = shrinker_debugfs_remove(shrinker); >>>> up_write(&shrinker_rwsem); >>>> + synchronize_srcu(&shrinker_srcu); >>>> + >>>> debugfs_remove_recursive(debugfs_entry); >>>> kfree(shrinker->nr_deferred); >>>> @@ -786,6 +789,7 @@ void synchronize_shrinkers(void) >>>> { >>>> down_write(&shrinker_rwsem); >>>> up_write(&shrinker_rwsem); >>>> + synchronize_srcu(&shrinker_srcu); >>>> } >>>> EXPORT_SYMBOL(synchronize_shrinkers); >>>> @@ -996,6 +1000,7 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >>>> { >>>> unsigned long ret, freed = 0; >>>> struct shrinker *shrinker; >>>> + int srcu_idx; >>>> /* >>>> * The root memcg might be allocated even though memcg is disabled >>>> @@ -1007,10 +1012,10 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >>>> if (!mem_cgroup_disabled() && !mem_cgroup_is_root(memcg)) >>>> return shrink_slab_memcg(gfp_mask, nid, memcg, priority); >>>> - if (!down_read_trylock(&shrinker_rwsem)) >>>> - goto out; >>>> + srcu_idx = srcu_read_lock(&shrinker_srcu); >>>> - list_for_each_entry(shrinker, &shrinker_list, list) { >>>> + list_for_each_entry_srcu(shrinker, &shrinker_list, list, >>>> + srcu_read_lock_held(&shrinker_srcu)) { >>>> struct shrink_control sc = { >>>> .gfp_mask = gfp_mask, >>>> .nid = nid, >>>> @@ -1021,19 +1026,9 @@ static unsigned long shrink_slab(gfp_t gfp_mask, int nid, >>>> if (ret == SHRINK_EMPTY) >>>> ret = 0; >>>> freed += ret; >>>> - /* >>>> - * Bail out if someone want to register a new shrinker to >>>> - * prevent the registration from being stalled for long periods >>>> - * by parallel ongoing shrinking. >>>> - */ >>>> - if (rwsem_is_contended(&shrinker_rwsem)) { >>>> - freed = freed ? : 1; >>>> - break; >>>> - } >>>> } >>>> - up_read(&shrinker_rwsem); >>>> -out: >>>> + srcu_read_unlock(&shrinker_srcu, srcu_idx); >>>> cond_resched(); >>>> return freed; >>>> } >>>> -- >>>> 2.20.1 >>>> >>>> >>> >>> Hi Qi, >>> >>> A different problem I realized after my old attempt to use SRCU was that the >>> unregister_shrinker() path became quite slow due to the heavy synchronize_srcu() >>> call. Both register_shrinker() *and* unregister_shrinker() are called frequently >>> these days, and SRCU is too unfair to the unregister path IMO. >> >> Hi Sultan, >> >> IIUC, for unregister_shrinker(), the wait time is hardly longer with >> SRCU than with shrinker_rwsem before. > > The wait time can be quite different because with shrinker_rwsem, the > rwsem_is_contended() bailout would cause unregister_shrinker() to wait for only > one random shrinker to finish at worst rather than waiting for *all* shrinkers > to finish.
Yes, to be exact, unregister_shrinker() needs to wait for all the shrinkers who entered grace period before it. But the benefit in exchange is that the slab shrink is completely lock-free, I think this is more worthwhile than letting unregister_shrinker() wait a little longer.
> >> And I just did a simple test. After using the script in cover letter to >> increase the shrink_slab hotspot, I did umount 1k times at the same >> time, and then I used bpftrace to measure the time consumption of >> unregister_shrinker() as follows: >> >> bpftrace -e 'kprobe:unregister_shrinker { @start[tid] = nsecs; } >> kretprobe:unregister_shrinker /@start[tid]/ { @ns[comm] = hist(nsecs - >> @start[tid]); delete(@start[tid]); }' >> >> @ns[umount]: >> [16K, 32K) 3 | | >> [32K, 64K) 66 |@@@@@@@@@@ | >> [64K, 128K) 32 |@@@@@ | >> [128K, 256K) 22 |@@@ | >> [256K, 512K) 48 |@@@@@@@ | >> [512K, 1M) 19 |@@@ | >> [1M, 2M) 131 |@@@@@@@@@@@@@@@@@@@@@ | >> [2M, 4M) 313 >> |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| >> [4M, 8M) 302 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ >> | >> [8M, 16M) 55 |@@@@@@@@@ >> >> I see that the highest time-consuming of unregister_shrinker() is between >> 8ms and 16ms, which feels tolerable? > > If you've got a fast x86 machine then I'd say that's a bit slow. :)
Nope, I tested it on a qemu virtual machine.
And I just tested it on a physical machine (Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz) and the results are as follows:
1) use synchronize_srcu():
@ns[umount]: [8K, 16K) 83 |@@@@@@@ | [16K, 32K) 578 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [32K, 64K) 78 |@@@@@@@ | [64K, 128K) 6 | | [128K, 256K) 7 | | [256K, 512K) 29 |@@ | [512K, 1M) 51 |@@@@ | [1M, 2M) 90 |@@@@@@@@ | [2M, 4M) 70 |@@@@@@ | [4M, 8M) 8 | |
2) use synchronize_srcu_expedited():
@ns[umount]: [8K, 16K) 31 |@@ | [16K, 32K) 803 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [32K, 64K) 158 |@@@@@@@@@@ | [64K, 128K) 4 | | [128K, 256K) 2 | | [256K, 512K) 2 | |
Thanks, Qi
> > This depends a lot on which shrinkers are active on your system and how much > work each one does upon running. If a driver's shrinker doesn't have much to do > because there's nothing it can shrink further, then it'll run fast. Conversely, > if a driver is stressed in a way that constantly creates a lot of potential work > for its shrinker, then the shrinker will run longer. > > Since shrinkers are allowed to sleep, the delays can really add up when waiting > for all of them to finish running. In the past, I recall observing delays of > 100ms+ in unregister_shrinker() on slower arm64 hardware when I stress tested > the SRCU approach. > > If your GPU driver has a shrinker (such as i915), I suggest testing again under > heavy GPU load. The GPU shrinkers can be pretty heavy IIRC. > > Thanks, > Sultan > >> Thanks, >> Qi >> >>> >>> Although I never got around to submitting it, I made a non-SRCU solution [1] >>> that uses fine-grained locking instead, which is fair to both the register path >>> and unregister path. (The patch I've linked is a version of this adapted to an >>> older 4.14 kernel FYI, but it can be reworked for the current kernel.) >>> >>> What do you think about the fine-grained locking approach? >>> >>> Thanks, >>> Sultan >>> >>> [1] https://github.com/kerneltoast/android_kernel_google_floral/commit/012378f3173a82d2333d3ae7326691544301e76a >>> >> >> -- >> Thanks, >> Qi
-- Thanks, Qi
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