Re: [PATCH] lowmemorykiller: Avoid excessive/redundant calling of LMK

[Michal Hocko]

On Mon 12-01-15 21:49:14, Chintan Pandya wrote:
> The global shrinker will invoke lowmem_shrink in a loop.
> The loop will be run (total_scan_pages/batch_size) times.
> The default batch_size will be 128 which will make
> shrinker invoking 100s of times. LMK does meaningful
> work only during first 2-3 times and then rest of the
> invocations are just CPU cycle waste. Fix that by returning
> to the shrinker with SHRINK_STOP when LMK doesn't find any
> more work to do. The deciding factor here is, no process
> found in the selected LMK bucket or memory conditions are
> sane.

lowmemory killer is broken by design and this one of the examples which
shows why. It simply doesn't fit into shrinkers concept.

The count_object callback simply lies and tells the core that all
the reclaimable LRU pages are scanable and gives it this as a number
which the core uses for total_scan. scan_objects callback then happily
ignore nr_to_reclaim and does its one time job where it iterates over
_all_ tasks and picks up the victim and returns its rss as a return
value. This is just a subset of LRU pages of course so it continues
looping until total_scan goes down to 0 finally.

If this really has to be a shrinker then, shouldn't it evaluate the OOM
situation in the count callback and return non zero only if OOM and then
the scan callback would kill and return nr_to_reclaim.

Or even better wouldn't it be much better to use vmpressure to wake
up a kernel module which would simply check the situation and kill
something?

Please do not put only cosmetic changes on top of broken concept and try
to think about a proper solution that is what staging is for AFAIU.

The code is in this state for quite some time and I would really hate if
it got merged just because it is in staging for too long and it is used
out there.

> Signed-off-by: Chintan Pandya <cpandya@xxxxxxxxxxxxxx>
> ---
>  drivers/staging/android/lowmemorykiller.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/staging/android/lowmemorykiller.c b/drivers/staging/android/lowmemorykiller.c
> index b545d3d..5bf483f 100644
> --- a/drivers/staging/android/lowmemorykiller.c
> +++ b/drivers/staging/android/lowmemorykiller.c
> @@ -110,7 +110,7 @@ static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
>  	if (min_score_adj == OOM_SCORE_ADJ_MAX + 1) {
>  		lowmem_print(5, "lowmem_scan %lu, %x, return 0\n",
>  			     sc->nr_to_scan, sc->gfp_mask);
> -		return 0;
> +		return SHRINK_STOP;
>  	}
>  
>  	selected_oom_score_adj = min_score_adj;
> @@ -163,6 +163,9 @@ static unsigned long lowmem_scan(struct shrinker *s, struct shrink_control *sc)
>  		set_tsk_thread_flag(selected, TIF_MEMDIE);
>  		send_sig(SIGKILL, selected, 0);
>  		rem += selected_tasksize;
> +	} else {
> +		rcu_read_unlock();
> +		return SHRINK_STOP;
>  	}
>  
>  	lowmem_print(4, "lowmem_scan %lu, %x, return %lu\n",
> -- 
> Chintan Pandya
> 
> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a
> member of the Code Aurora Forum, hosted by The Linux Foundation
> 
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-- 
Michal Hocko
SUSE Labs
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