From: Muchun Song <songmuchun@bytedance.com> To: Michal Hocko <mhocko@suse.com> Cc: Johannes Weiner <hannes@cmpxchg.org>, Vladimir Davydov <vdavydov.dev@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Cgroups <cgroups@vger.kernel.org>, Linux Memory Management List <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org> Subject: Re: [External] Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero Date: Mon, 15 Feb 2021 19:34:13 +0800 [thread overview] Message-ID: <CAMZfGtUD3H6e+s_n+2q9aE3ABKJaooRj_vyELBaTTVUssSK-NA@mail.gmail.com> (raw) In-Reply-To: <YCpMo6gLFyqANsgd@dhcp22.suse.cz> On Mon, Feb 15, 2021 at 6:27 PM Michal Hocko <mhocko@suse.com> wrote: > > On Mon 15-02-21 18:09:44, Muchun Song wrote: > > On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko@suse.com> wrote: > > > > > > On Sat 13-02-21 01:01:58, Muchun Song wrote: > > > > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > > > > so idr_find() is pointless and wastes CPU cycles. > > > > > > Is this possible at all to happen? If not why should we add a test for > > > _all_ invocations? > > > > Yeah, this indeed can happen. If we allocate a new swap cache page > > and charge it via mem_cgroup_charge, then the page will uncharge > > the swap counter via mem_cgroup_uncharge_swap. When the swap > > entry is indeed freed, we will call mem_cgroup_uncharge_swap again, > > In this routine, we can pass zero to mem_cgroup_from_id. Right? > > If the above claim is correct, which I would need to double check then > it should have been part of the changelog! Please think of your poor > reviewers and the time they have to invest into the review. The easy way may be adding a printk to mem_cgroup_from_id when the parameter is zero. > > I would also like to see your waste of CPU cycles argument to be backed > by something. Are we talking about cycles due to an additional function Yeah, when the parameter is already zero, idr_find() must return zero. So I think that the additional function call is unnecessary. I have added a printk to mem_cgroup_from_id, I found the parameter can be zero several times. > call? Is this really something we should even care about? Maybe not. Just my thoughts. Thanks. > > > > > > > > > Signed-off-by: Muchun Song <songmuchun@bytedance.com> > > > > --- > > > > mm/memcontrol.c | 3 +++ > > > > 1 file changed, 3 insertions(+) > > > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > > index a3f26522765a..68ed4b297c13 100644 > > > > --- a/mm/memcontrol.c > > > > +++ b/mm/memcontrol.c > > > > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > > > > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > > > > { > > > > WARN_ON_ONCE(!rcu_read_lock_held()); > > > > + /* The memcg ID cannot be zero. */ > > > > + if (id == 0) > > > > + return NULL; > > > > return idr_find(&mem_cgroup_idr, id); > > > > } > > > > > > > > -- > > > > 2.11.0 > > > > > > -- > > > Michal Hocko > > > SUSE Labs > > -- > Michal Hocko > SUSE Labs
WARNING: multiple messages have this Message-ID (diff)
From: Muchun Song <songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org> To: Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> Cc: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>, Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Linux Memory Management List <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org> Subject: Re: [External] Re: [PATCH 3/4] mm: memcontrol: bail out early when id is zero Date: Mon, 15 Feb 2021 19:34:13 +0800 [thread overview] Message-ID: <CAMZfGtUD3H6e+s_n+2q9aE3ABKJaooRj_vyELBaTTVUssSK-NA@mail.gmail.com> (raw) In-Reply-To: <YCpMo6gLFyqANsgd-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org> On Mon, Feb 15, 2021 at 6:27 PM Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> wrote: > > On Mon 15-02-21 18:09:44, Muchun Song wrote: > > On Mon, Feb 15, 2021 at 5:39 PM Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org> wrote: > > > > > > On Sat 13-02-21 01:01:58, Muchun Song wrote: > > > > The memcg ID cannot be zero, but we can pass zero to mem_cgroup_from_id, > > > > so idr_find() is pointless and wastes CPU cycles. > > > > > > Is this possible at all to happen? If not why should we add a test for > > > _all_ invocations? > > > > Yeah, this indeed can happen. If we allocate a new swap cache page > > and charge it via mem_cgroup_charge, then the page will uncharge > > the swap counter via mem_cgroup_uncharge_swap. When the swap > > entry is indeed freed, we will call mem_cgroup_uncharge_swap again, > > In this routine, we can pass zero to mem_cgroup_from_id. Right? > > If the above claim is correct, which I would need to double check then > it should have been part of the changelog! Please think of your poor > reviewers and the time they have to invest into the review. The easy way may be adding a printk to mem_cgroup_from_id when the parameter is zero. > > I would also like to see your waste of CPU cycles argument to be backed > by something. Are we talking about cycles due to an additional function Yeah, when the parameter is already zero, idr_find() must return zero. So I think that the additional function call is unnecessary. I have added a printk to mem_cgroup_from_id, I found the parameter can be zero several times. > call? Is this really something we should even care about? Maybe not. Just my thoughts. Thanks. > > > > > > > > > Signed-off-by: Muchun Song <songmuchun-EC8Uxl6Npydl57MIdRCFDg@public.gmane.org> > > > > --- > > > > mm/memcontrol.c | 3 +++ > > > > 1 file changed, 3 insertions(+) > > > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > > index a3f26522765a..68ed4b297c13 100644 > > > > --- a/mm/memcontrol.c > > > > +++ b/mm/memcontrol.c > > > > @@ -5173,6 +5173,9 @@ static inline void mem_cgroup_id_put(struct mem_cgroup *memcg) > > > > struct mem_cgroup *mem_cgroup_from_id(unsigned short id) > > > > { > > > > WARN_ON_ONCE(!rcu_read_lock_held()); > > > > + /* The memcg ID cannot be zero. */ > > > > + if (id == 0) > > > > + return NULL; > > > > return idr_find(&mem_cgroup_idr, id); > > > > } > > > > > > > > -- > > > > 2.11.0 > > > > > > -- > > > Michal Hocko > > > SUSE Labs > > -- > Michal Hocko > SUSE Labs
next prev parent reply other threads:[~2021-02-15 11:35 UTC|newest] Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-02-12 17:01 [PATCH 1/4] mm: memcontrol: remove memcg check from memcg_oom_recover Muchun Song 2021-02-12 17:01 ` Muchun Song 2021-02-12 17:01 ` [PATCH 2/4] mm: memcontrol: add missing memcg_oom_recover() when uncharge slab page Muchun Song 2021-02-12 17:01 ` Muchun Song 2021-02-15 9:37 ` Michal Hocko 2021-02-15 9:37 ` Michal Hocko 2021-02-12 17:01 ` [PATCH 3/4] mm: memcontrol: bail out early when id is zero Muchun Song 2021-02-12 17:01 ` Muchun Song 2021-02-15 9:39 ` Michal Hocko 2021-02-15 9:39 ` Michal Hocko 2021-02-15 10:09 ` [External] " Muchun Song 2021-02-15 10:09 ` Muchun Song 2021-02-15 10:09 ` Muchun Song 2021-02-15 10:27 ` Michal Hocko 2021-02-15 10:27 ` Michal Hocko 2021-02-15 11:34 ` Muchun Song [this message] 2021-02-15 11:34 ` Muchun Song 2021-02-15 11:34 ` Muchun Song 2021-02-12 17:01 ` [PATCH 4/4] mm: memcontrol: fix swap uncharge on cgroup v2 Muchun Song 2021-02-12 17:01 ` Muchun Song 2021-02-12 18:56 ` Shakeel Butt 2021-02-12 18:56 ` Shakeel Butt 2021-02-13 6:48 ` [External] " Muchun Song 2021-02-13 6:48 ` Muchun Song 2021-02-16 17:16 ` Shakeel Butt 2021-02-16 17:16 ` Shakeel Butt 2021-02-16 17:16 ` Shakeel Butt 2021-02-15 9:47 ` Michal Hocko 2021-02-15 9:47 ` Michal Hocko 2021-02-15 10:15 ` [External] " Muchun Song 2021-02-15 10:15 ` Muchun Song 2021-02-15 10:15 ` Muchun Song 2021-02-15 10:24 ` Michal Hocko 2021-02-15 10:24 ` Michal Hocko 2021-02-16 16:59 ` Johannes Weiner 2021-02-16 17:17 ` Shakeel Butt 2021-02-16 17:17 ` Shakeel Butt 2021-02-16 17:17 ` Shakeel Butt 2021-02-16 17:19 ` Michal Hocko 2021-02-16 17:19 ` Michal Hocko 2021-02-16 17:28 ` Johannes Weiner 2021-02-16 17:28 ` Johannes Weiner 2021-02-17 5:15 ` [External] " Muchun Song 2021-02-17 5:15 ` Muchun Song 2021-02-17 5:15 ` Muchun Song 2021-02-17 5:11 ` Muchun Song 2021-02-17 5:11 ` Muchun Song 2021-02-17 5:11 ` Muchun Song 2021-02-15 9:24 ` [PATCH 1/4] mm: memcontrol: remove memcg check from memcg_oom_recover Michal Hocko 2021-02-15 9:24 ` Michal Hocko
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