Re: [PATCH] mm: Sync percpu mm RSS counters before querying

["Michal Koutný" <mkoutny@suse.com>]

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On Fri, Jun 16, 2023 at 12:31:44PM -0700, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Fri, 16 Jun 2023 20:07:18 +0200 Michal Koutný <mkoutny@suse.com> wrote:
> 
> > An issue was observed with stats collected in struct rusage on ppc64le
> > with 64kB pages. The percpu counters use batching with
> > 	percpu_counter_batch = max(32, nr*2) # in PAGE_SIZE
> > i.e. with larger pages but similar RSS consumption (bytes), there'll be
> > less flushes and error more noticeable.
> 
> A fully detailed description of the issue would be helpful.  Obviously
> "inaccuracy", but how bad?

Any reader of get_mm_counter() could see the inaccuracy given by the
formula.
In this particular case it is detected by a testsuite of time(1) utility
that feeds from rusage:

> FAIL: tests/time-max-rss
> ========================
> 
> time(1) failed to detect 5MB allcoation.
>   mem-baseline(kb): 0
>   mem-5MB(kb):      4096
>   delta(kb):        4096
> FAIL tests/time-max-rss.sh (exit status: 1)

(i.e. 1MB missing)

> Far too large to be inlined!  For six callsites it adds 1kb of text.

Ah, thanks, I can change that.

> Why even modify the counter?  Can't <whatever this issue is> be
> addressed by using percpu_counter_sum() in an appropriate place?

I considered modifying get_mm_counter(), however, I decided not to put
the per-cpu summing there as it'd incur the impact to many more places
than sync_mm_rss().

> For unknown reasons percpu_counter_set() uses for_each_possible_cpu(). 
> Probably just a mistake - percpu_counters are hotplug-aware and
> for_each_online_cpu should suffice.

Yeah, that could be cleaned up in another patch (cf mask in
__percpu_counter_sum).

> I'm really not liking percpu_counter_set().  It's only safe in
> situations where the caller knows that no other CPU can be modifying
> the counter.  I wonder if all the callers know that.

I admit I only considered the do_exit() path (and even that isn't
granted in a multithreaded process) -- so I don't like
percpu_counter_set() in this current form neither.
I will need to review effects of parallel updates more.

Thanks,
Michal

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