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Date: Fri, 18 Nov 2011 15:08:35 +0800
From: Yong Zhang <yong.zhang0@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>,
LKML <linux-kernel@...r.kernel.org>,
linux-rt-users <linux-rt-users@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH -rt] memcg: use migrate_disable()/migrate_enable( ) in
memcg_check_events()
On Thu, Nov 17, 2011 at 11:25:56AM +0100, Thomas Gleixner wrote:
> On Thu, 17 Nov 2011, Yong Zhang wrote:
>
> > On Wed, Nov 16, 2011 at 06:02:42PM +0100, Thomas Gleixner wrote:
> > > On Wed, 16 Nov 2011, Steven Rostedt wrote:
> > > > On Wed, 2011-11-16 at 17:16 +0800, Yong Zhang wrote:
> > > > > Looking at commit 4799401f [memcg: Fix race condition in
> > > > > memcg_check_events() with this_cpu usage], we just want
> > > > > to disable migration. So use the right API in -rt. This
> > > > > will cure below warning.
> > > > No this won't work. Not even for -rt. If we disable migration but not
> > > > preemption, then two tasks can take this path. And the checks in
> > > > __memcg_event_check() will be corrupted because nothing is protecting
> > > > the updates from two tasks going into the same path.
> > > >
> > > > Perhaps a local_lock would work.
> > >
> > > Yes, that's the only sensible option for now. Untested patch below.
> >
> > Works for me.
>
> Johannes came up with a different solution. Could you please give it a try?
Works too :)
Thanks,
Yong
>
> Thanks,
>
> tglx
>
> ------------->
> Subject: [patch] mm: memcg: shorten preempt-disabled section around event checks
>
> Only the ratelimit checks themselves have to run with preemption
> disabled, the resulting actions - checking for usage thresholds,
> updating the soft limit tree - can and should run with preemption
> enabled.
>
> Signed-off-by: Johannes Weiner <jweiner@...hat.com>
> ---
> mm/memcontrol.c | 73 ++++++++++++++++++++++++++----------------------------
> 1 files changed, 35 insertions(+), 38 deletions(-)
>
> Thomas, HTH and it is probably interesting for upstream as well.
> Unfortunately, I'm in the middle of moving right now, so this is
> untested except for compiling.
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6aff93c..8e62d3e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -683,37 +683,32 @@ static unsigned long mem_cgroup_nr_lru_pages(struct mem_cgroup *memcg,
> return total;
> }
>
> -static bool __memcg_event_check(struct mem_cgroup *memcg, int target)
> +static bool mem_cgroup_event_ratelimit(struct mem_cgroup *memcg,
> + enum mem_cgroup_events_target target)
> {
> unsigned long val, next;
>
> val = __this_cpu_read(memcg->stat->events[MEM_CGROUP_EVENTS_COUNT]);
> next = __this_cpu_read(memcg->stat->targets[target]);
> /* from time_after() in jiffies.h */
> - return ((long)next - (long)val < 0);
> -}
> -
> -static void __mem_cgroup_target_update(struct mem_cgroup *memcg, int target)
> -{
> - unsigned long val, next;
> -
> - val = __this_cpu_read(memcg->stat->events[MEM_CGROUP_EVENTS_COUNT]);
> -
> - switch (target) {
> - case MEM_CGROUP_TARGET_THRESH:
> - next = val + THRESHOLDS_EVENTS_TARGET;
> - break;
> - case MEM_CGROUP_TARGET_SOFTLIMIT:
> - next = val + SOFTLIMIT_EVENTS_TARGET;
> - break;
> - case MEM_CGROUP_TARGET_NUMAINFO:
> - next = val + NUMAINFO_EVENTS_TARGET;
> - break;
> - default:
> - return;
> + if ((long)next - (long)val < 0) {
> + switch (target) {
> + case MEM_CGROUP_TARGET_THRESH:
> + next = val + THRESHOLDS_EVENTS_TARGET;
> + break;
> + case MEM_CGROUP_TARGET_SOFTLIMIT:
> + next = val + SOFTLIMIT_EVENTS_TARGET;
> + break;
> + case MEM_CGROUP_TARGET_NUMAINFO:
> + next = val + NUMAINFO_EVENTS_TARGET;
> + break;
> + default:
> + break;
> + }
> + __this_cpu_write(memcg->stat->targets[target], next);
> + return true;
> }
> -
> - __this_cpu_write(memcg->stat->targets[target], next);
> + return false;
> }
>
> /*
> @@ -724,25 +719,27 @@ static void memcg_check_events(struct mem_cgroup *memcg, struct page *page)
> {
> preempt_disable();
> /* threshold event is triggered in finer grain than soft limit */
> - if (unlikely(__memcg_event_check(memcg, MEM_CGROUP_TARGET_THRESH))) {
> + if (unlikely(mem_cgroup_event_ratelimit(memcg,
> + MEM_CGROUP_TARGET_THRESH))) {
> + bool do_softlimit, do_numainfo;
> +
> + do_softlimit = mem_cgroup_event_ratelimit(memcg,
> + MEM_CGROUP_TARGET_SOFTLIMIT);
> +#if MAX_NUMNODES > 1
> + do_numainfo = mem_cgroup_event_ratelimit(memcg,
> + MEM_CGROUP_TARGET_NUMAINFO);
> +#endif
> + preempt_enable();
> +
> mem_cgroup_threshold(memcg);
> - __mem_cgroup_target_update(memcg, MEM_CGROUP_TARGET_THRESH);
> - if (unlikely(__memcg_event_check(memcg,
> - MEM_CGROUP_TARGET_SOFTLIMIT))) {
> + if (unlikely(do_softlimit))
> mem_cgroup_update_tree(memcg, page);
> - __mem_cgroup_target_update(memcg,
> - MEM_CGROUP_TARGET_SOFTLIMIT);
> - }
> #if MAX_NUMNODES > 1
> - if (unlikely(__memcg_event_check(memcg,
> - MEM_CGROUP_TARGET_NUMAINFO))) {
> + if (unlikely(do_numainfo))
> atomic_inc(&memcg->numainfo_events);
> - __mem_cgroup_target_update(memcg,
> - MEM_CGROUP_TARGET_NUMAINFO);
> - }
> #endif
> - }
> - preempt_enable();
> + } else
> + preempt_enable();
> }
>
> static struct mem_cgroup *mem_cgroup_from_cont(struct cgroup *cont)
> --
> 1.7.6.4
>
> --
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