Re: [PATCH v3 RFC] mm/vmscan: more restrictive condition for retry of shrink_zones

[Shakeel Butt]

On Mon, Mar 13, 2017 at 1:33 AM, Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> Please do not post new version after a single feedback and try to wait
> for more review to accumulate. This is in the 3rd version and it is not
> clear why it is still an RFC.
>
> On Sun 12-03-17 19:06:10, Yisheng Xie wrote:
>> From: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
>>
>> When we enter do_try_to_free_pages, the may_thrash is always clear, and
>> it will retry shrink zones to tap cgroup's reserves memory by setting
>> may_thrash when the former shrink_zones reclaim nothing.
>>
>> However, when memcg is disabled or on legacy hierarchy, it should not do
>> this useless retry at all, for we do not have any cgroup's reserves
>> memory to tap, and we have already done hard work but made no progress.
>>
>> To avoid this time costly and useless retrying, add a stub function
>> mem_cgroup_thrashed() and return true when memcg is disabled or on
>> legacy hierarchy.
>
> Have you actually seen this as a bad behavior? On which workload? Or
> have spotted this by the code review?
>
> Please note that more than _what_ it is more interesting _why_ the patch
> has been prepared.
>
> I agree the current additional round of reclaim is just lame because we
> are trying hard to control the retry logic from the page allocator which
> is a sufficient justification to fix this IMO. But I really hate the
> name. At this point we do not have any idea that the memcg is trashing
> as the name of the function suggests.
>
> All of them simply might not have any reclaimable pages. So I would
> suggest either a better name e.g. memcg_allow_lowmem_reclaim() or,
> preferably, fix this properly. E.g. something like the following.
> ---
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bae698484e8e..989ba9761921 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -99,6 +99,9 @@ struct scan_control {
>         /* Can cgroups be reclaimed below their normal consumption range? */
>         unsigned int may_thrash:1;
>
> +       /* Did we have any memcg protected by the low limit */
> +       unsigned int memcg_low_protection:1;
> +
>         unsigned int hibernation_mode:1;
>
>         /* One of the zones is ready for compaction */
> @@ -2513,6 +2516,7 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>                         if (mem_cgroup_low(root, memcg)) {
>                                 if (!sc->may_thrash)
>                                         continue;
> +                               sc->memcg_low_protection = true;

I think you wanted to put this statement before the continue otherwise
it will just disable the sc->may_thrash (second reclaim pass)
altogether.

>                                 mem_cgroup_events(memcg, MEMCG_LOW, 1);
>                         }
>
> @@ -2774,7 +2778,7 @@ static unsigned long do_try_to_free_pages(struct zonelist *zonelist,
>                 return 1;
>
>         /* Untapped cgroup reserves?  Don't OOM, retry. */
> -       if (!sc->may_thrash) {
> +       if ( sc->memcg_low_protection && !sc->may_thrash) {
>                 sc->priority = initial_priority;
>                 sc->may_thrash = 1;
>                 goto retry;
> --
> Michal Hocko
> SUSE Labs