Re: [PATCH] Synchronize task mm counters on context switch

[Minchan Kim <minchan@kernel.org>]

On Wed, Feb 21, 2018 at 04:23:43PM -0800, Daniel Colascione wrote:
> Thanks for taking a look.
> 
> On Wed, Feb 21, 2018 at 4:16 PM, Minchan Kim <minchan@kernel.org> wrote:
> 
> > Hi Daniel,
> >
> > On Wed, Feb 21, 2018 at 11:05:04AM -0800, Daniel Colascione wrote:
> > > On Mon, Feb 5, 2018 at 2:03 PM, Daniel Colascione <dancol@google.com>
> > wrote:
> > >
> > > > When SPLIT_RSS_COUNTING is in use (which it is on SMP systems,
> > > > generally speaking), we buffer certain changes to mm-wide counters
> > > > through counters local to the current struct task, flushing them to
> > > > the mm after seeing 64 page faults, as well as on task exit and
> > > > exec. This scheme can leave a large amount of memory unaccounted-for
> > > > in process memory counters, especially for processes with many threads
> > > > (each of which gets 64 "free" faults), and it produces an
> > > > inconsistency with the same memory counters scanned VMA-by-VMA using
> > > > smaps. This inconsistency can persist for an arbitrarily long time,
> > > > since there is no way to force a task to flush its counters to its mm.
> >
> > Nice catch. Incosistency is bad but we usually have done it for
> > performance.
> > So, FWIW, it would be much better to describe what you are suffering from
> > for matainter to take it.
> >
> 
> The problem is that the per-process counters in /proc/pid/status lag behind
> the actual memory allocations, leading to an inaccurate view of overall
> memory consumed by each process.

Yub, true. The key of question was why you need a such accurate count.
Don't get me wrong. I'm not saying we don't need it.
I was just curious why it becomes important now because we have been with
such inaccurate count for a decade. 

> 
> 
> > > > This patch flushes counters on context switch. This way, we bound the
> > > > amount of unaccounted memory without forcing tasks to flush to the
> > > > mm-wide counters on each minor page fault. The flush operation should
> > > > be cheap: we only have a few counters, adjacent in struct task, and we
> > > > don't atomically write to the mm counters unless we've changed
> > > > something since the last flush.
> > > >
> > > > Signed-off-by: Daniel Colascione <dancol@google.com>
> > > > ---
> > > >  kernel/sched/core.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > > index a7bf32aabfda..7f197a7698ee 100644
> > > > --- a/kernel/sched/core.c
> > > > +++ b/kernel/sched/core.c
> > > > @@ -3429,6 +3429,9 @@ asmlinkage __visible void __sched schedule(void)
> > > >         struct task_struct *tsk = current;
> > > >
> > > >         sched_submit_work(tsk);
> > > > +       if (tsk->mm)
> > > > +               sync_mm_rss(tsk->mm);
> > > > +
> > > >         do {
> > > >                 preempt_disable();
> > > >                 __schedule(false);
> > > >
> > >
> > >
> > > Ping? Is this approach just a bad idea? We could instead just manually
> > sync
> > > all mm-attached tasks at counter-retrieval time.
> >
> > IMHO, yes, it should be done when user want to see which would be really
> > cold path while this shecule function is hot.
> >
> 
> The problem with doing it that way is that we need to look at each task
> attached to a particular mm. AFAIK (and please tell me if I'm wrong), the
> only way to do that is to iterate over all processes, and for each process
> attached to the mm we want, iterate over all its tasks (since each one has
> to have the same mm, I think). Does that sound right?

Hmm, it seems you're right. I spent some time to think over but cannot reach
a better idea. One of option was to change RSS_EVENT_THRESH to per-mm and
control it dynamically with the count of mm_users when forking time.
However, it makes the process with many thread harmful without reason.

So, I support your idea at this moment. But let's hear other's opinions.

Thanks.

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