Re: [PATCH v4] mm/vmscan: more restrictive condition for retry in do_try_to_free_pages

[Michal Hocko]

On Wed 15-03-17 19:36:48, Yisheng Xie wrote:
> By reviewing code, I find that when 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, or there do not
> have any memcg protected by low limit, 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 unneeded retrying, add a new field in scan_control named
> memcg_low_protection, set it if there is any memcg protected by low limit
> and only do the retry when memcg_low_protection is set while may_thrash
> is clear.

You still haven't explained why a retry is bad thing. It certainly is
not about performance because not a single page being reclaimed means
that all the performance went to hell already. Please always make it
clear why the change is needed/desirable.

But I agree that this makes the code easier to understand so I am OK
with this change.

> Signed-off-by: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
> Suggested-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
> v4:
>  - add a new field in scan_control named memcg_low_protection to check whether
>    there have any memcg protected by low limit. - Michal
> 
> v3:
>  - rename function may_thrash() to mem_cgroup_thrashed() to avoid confusing.
> 
> v2:
>  - more restrictive condition for retry of shrink_zones (restricting
>    cgroup_disabled=memory boot option and cgroup legacy hierarchy) - Shakeel
> 
>  - add a stub function may_thrash() to avoid compile error or warning.
> 
>  - rename subject from "donot retry shrink zones when memcg is disable"
>    to "more restrictive condition for retry in do_try_to_free_pages"
> 
> Any comment is more than welcome!
> 
> Thanks
> Yisheng Xie
> 
>  mm/vmscan.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index bc8031e..c4fa3d3 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -100,6 +100,9 @@ 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 */
> @@ -2557,6 +2560,8 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
>  			unsigned long scanned;
>  
>  			if (mem_cgroup_low(root, memcg)) {
> +				sc->memcg_low_protection = 1;
> +
>  				if (!sc->may_thrash)
>  					continue;
>  				mem_cgroup_events(memcg, MEMCG_LOW, 1);
> @@ -2808,7 +2813,7 @@ 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;
> -- 
> 1.7.12.4
> 

-- 
Michal Hocko
SUSE Labs