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Date: Fri, 24 Feb 2023 18:12:31 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Sultan Alsawaf <sultan@...neltoast.com>
Cc: akpm@...ux-foundation.org, tkhai@...ru, hannes@...xchg.org,
shakeelb@...gle.com, mhocko@...nel.org, roman.gushchin@...ux.dev,
muchun.song@...ux.dev, david@...hat.com, shy828301@...il.com,
dave@...olabs.net, penguin-kernel@...ove.sakura.ne.jp,
paulmck@...nel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] mm: vmscan: make global slab shrink lockless
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@...edance.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 /@...rt[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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