From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: ARC-Seal: i=1; a=rsa-sha256; t=1524567384; cv=none; d=google.com; s=arc-20160816; b=E+lnV+qL3UkGSzBzkZdi/eiKLabm8cUbn41gNHGKk2wYtNkw8zq5rUFFBqdiPkfrf4 EI6oSn0xqMbhr/ObAVjQuAygu7G9U2WMggLr/szX/ll7Tey+4Eefm0ONSJG+vFRxxnXY AQA/Ppzm45tURuPIysVH17GSxW53lfrnLhpg0MU7y+kQgJdA352xXmI8ylX7A8E5u4ga qwdvTBPn7HHSlsZFzFfwyyMEOalEYVM0sW9grFqvoyw345w53br60/T94VePESiC9BhU bikCXTztlAFn9uxkqU7Cl5uTymNxzp+ggYE5qEpQsogGySlhP4R064ey9IYpDLxnbDIG hqTA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:dkim-signature:arc-authentication-results; bh=5MA0JF9fTfRQFfLfayNyTt/oCXyYIfZH4e083MWlONA=; b=UY4Z99UkdA+U4MBYKeMEss/pScnaKwxIJ8Ll6NrcPNNg3GZOHwTRsurghYCQdRI9tv 2xXq0qNktMrba/csoKSrQ5Z7FOTRrYHt3bCP+Jv/RsCi6QxKxliJTBZlRAV/ko58WNam RET+JzDWAz24xnhDwAU4S5P5U1IU2vdPwHGEC+XuHNshaulm0awbPZCZfO66ofWWWBGW 1Xy5sTYzgKeGTkVtQ2SGeM8CWIpuBfnEheNBsN9kA7BlzgXu2tFF6G7PnBQSK8CgM2nO SvfHamPbfS7EdcDr6nL5HJuXeCBqOBBj+exESc+1pLQTK9xCmAAC0RAETUsd6KlGIbQ0 3tyA== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=E/M6G0wL; spf=pass (google.com: domain of vdavydov.dev@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=vdavydov.dev@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=E/M6G0wL; spf=pass (google.com: domain of vdavydov.dev@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=vdavydov.dev@gmail.com; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com X-Google-Smtp-Source: AIpwx4+IA74ZWeC4kUVIARGQ+d9dKIzjNsi440YISWOIyJyKr/jMP12Sz65WSBK/UfNadoA4FeRz4A== Date: Tue, 24 Apr 2018 13:56:20 +0300 From: Vladimir Davydov To: Kirill Tkhai Cc: akpm@linux-foundation.org, shakeelb@google.com, viro@zeniv.linux.org.uk, hannes@cmpxchg.org, mhocko@kernel.org, tglx@linutronix.de, pombredanne@nexb.com, stummala@codeaurora.org, gregkh@linuxfoundation.org, sfr@canb.auug.org.au, guro@fb.com, mka@chromium.org, penguin-kernel@I-love.SAKURA.ne.jp, chris@chris-wilson.co.uk, longman@redhat.com, minchan@kernel.org, hillf.zj@alibaba-inc.com, ying.huang@intel.com, mgorman@techsingularity.net, jbacik@fb.com, linux@roeck-us.net, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@infradead.org, lirongqing@baidu.com, aryabinin@virtuozzo.com Subject: Re: [PATCH v2 12/12] mm: Clear shrinker bit if there are no objects related to memcg Message-ID: <20180424105238.6quxqngkmtok7ghn@esperanza> References: <152397794111.3456.1281420602140818725.stgit@localhost.localdomain> <152399129187.3456.5685999465635300270.stgit@localhost.localdomain> <20180422182132.c4tqkyy4ojgi7l7q@esperanza> <17b76fd4-ce80-50cf-6149-1f3908081ae7@virtuozzo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <17b76fd4-ce80-50cf-6149-1f3908081ae7@virtuozzo.com> X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1598009382637676470?= X-GMAIL-MSGID: =?utf-8?q?1598624769412833793?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Mon, Apr 23, 2018 at 01:01:08PM +0300, Kirill Tkhai wrote: > On 22.04.2018 21:21, Vladimir Davydov wrote: > > On Tue, Apr 17, 2018 at 09:54:51PM +0300, Kirill Tkhai wrote: > >> To avoid further unneed calls of do_shrink_slab() > >> for shrinkers, which already do not have any charged > >> objects in a memcg, their bits have to be cleared. > >> > >> This patch introduces a lockless mechanism to do that > >> without races without parallel list lru add. After > >> do_shrink_slab() returns SHRINK_EMPTY the first time, > >> we clear the bit and call it once again. Then we restore > >> the bit, if the new return value is different. > >> > >> Note, that single smp_mb__after_atomic() in shrink_slab_memcg() > >> covers two situations: > >> > >> 1)list_lru_add() shrink_slab_memcg > >> list_add_tail() for_each_set_bit() <--- read bit > >> do_shrink_slab() <--- missed list update (no barrier) > >> > >> set_bit() do_shrink_slab() <--- seen list update > >> > >> This situation, when the first do_shrink_slab() sees set bit, > >> but it doesn't see list update (i.e., race with the first element > >> queueing), is rare. So we don't add before the first call > >> of do_shrink_slab() instead of this to do not slow down generic > >> case. Also, it's need the second call as seen in below in (2). > >> > >> 2)list_lru_add() shrink_slab_memcg() > >> list_add_tail() ... > >> set_bit() ... > >> ... for_each_set_bit() > >> do_shrink_slab() do_shrink_slab() > >> clear_bit() ... > >> ... ... > >> list_lru_add() ... > >> list_add_tail() clear_bit() > >> > >> set_bit() do_shrink_slab() > >> > >> The barriers guarantees, the second do_shrink_slab() > >> in the right side task sees list update if really > >> cleared the bit. This case is drawn in the code comment. > >> > >> [Results/performance of the patchset] > >> > >> After the whole patchset applied the below test shows signify > >> increase of performance: > >> > >> $echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy > >> $mkdir /sys/fs/cgroup/memory/ct > >> $echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes > >> $for i in `seq 0 4000`; do mkdir /sys/fs/cgroup/memory/ct/$i; echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs; mkdir -p s/$i; mount -t tmpfs $i s/$i; touch s/$i/file; done > >> > >> Then, 4 sequential calls of drop caches: > >> $time echo 3 > /proc/sys/vm/drop_caches > >> > >> 1)Before: > >> 0.00user 8.99system 0:08.99elapsed 99%CPU > >> 0.00user 5.97system 0:05.97elapsed 100%CPU > >> 0.00user 5.97system 0:05.97elapsed 100%CPU > >> 0.00user 5.85system 0:05.85elapsed 100%CPU > >> > >> 2)After > >> 0.00user 1.11system 0:01.12elapsed 99%CPU > >> 0.00user 0.00system 0:00.00elapsed 100%CPU > >> 0.00user 0.00system 0:00.00elapsed 100%CPU > >> 0.00user 0.00system 0:00.00elapsed 100%CPU > >> > >> Even if we round 0:00.00 up to 0:00.01, the results shows > >> the performance increases at least in 585 times. > >> > >> Signed-off-by: Kirill Tkhai > >> --- > >> include/linux/memcontrol.h | 2 ++ > >> mm/vmscan.c | 19 +++++++++++++++++-- > >> 2 files changed, 19 insertions(+), 2 deletions(-) > >> > >> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h > >> index e1c1fa8e417a..1c5c68550e2f 100644 > >> --- a/include/linux/memcontrol.h > >> +++ b/include/linux/memcontrol.h > >> @@ -1245,6 +1245,8 @@ static inline void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int nr) > >> > >> rcu_read_lock(); > >> map = SHRINKERS_MAP(memcg, nid); > >> + /* Pairs with smp mb in shrink_slab() */ > >> + smp_mb__before_atomic(); > >> set_bit(nr, map->map); > >> rcu_read_unlock(); > >> } > >> diff --git a/mm/vmscan.c b/mm/vmscan.c > >> index 3be9b4d81c13..a8733bc5377b 100644 > >> --- a/mm/vmscan.c > >> +++ b/mm/vmscan.c > >> @@ -579,8 +579,23 @@ static unsigned long shrink_slab_memcg(gfp_t gfp_mask, int nid, > >> } > >> > >> ret = do_shrink_slab(&sc, shrinker, priority); > >> - if (ret == SHRINK_EMPTY) > >> - ret = 0; > >> + if (ret == SHRINK_EMPTY) { > >> + clear_bit(i, map->map); > >> + /* > >> + * Pairs with mb in set_shrinker_bit(): > >> + * > >> + * list_lru_add() shrink_slab_memcg() > >> + * list_add_tail() clear_bit() > >> + * > >> + * set_bit() do_shrink_slab() > >> + */ > >> + smp_mb__after_atomic(); > >> + ret = do_shrink_slab(&sc, shrinker, priority); > >> + if (ret == SHRINK_EMPTY) > >> + ret = 0; > >> + else > >> + set_shrinker_bit(memcg, nid, i); > >> + } > > > > This is mind-boggling. Are there any alternatives? For instance, can't > > we clear the bit in list_lru_del, when we hold the list lock? > > Since a single shrinker may iterate over several lru lists, we can't do that. > Otherwise, we would have to probe another shrinker's lru list from a lru list, > which became empty in list_lru_del(). > > The solution I suggested, is generic, and it does not depend on low-level > structure type, used by shrinker. This even doesn't have to be a lru list. Fair enough. I guess this is the best way to go after all. Please try to improve the comment so that it isn't just a pure diagram. Also, please prefix all memcg-related function names (such as set_shrinker_bit) with memcg_ (or mem_cgroup_) in this and all other patches.