Re: [PATCH v1] mm/vmscan: Add retry logic for cgroups with memory.low in kswapd

[Michal Hocko <mhocko@suse.com>]

On Tue 14-10-25 16:18:49, Jiayuan Chen wrote:
> We can set memory.low for cgroups as a soft protection limit. When the
> kernel cannot reclaim any pages from other cgroups, it retries reclaim
> while ignoring the memory.low protection of the skipped cgroups.
> 
> Currently, this retry logic only works in direct reclaim path, but is
> missing in the kswapd asynchronous reclaim. Typically, a cgroup may
> contain some cold pages that could be reclaimed even when memory.low is
> set.
> 
> This change adds retry logic to kswapd: if the first reclaim attempt fails
> to reclaim any pages and some cgroups were skipped due to memory.low
> protection, kswapd will perform a second reclaim pass ignoring memory.low
> restrictions.
> 
> This ensures more consistent reclaim behavior between direct reclaim and
> kswapd. By allowing kswapd to reclaim more proactively from protected
> cgroups under global memory pressure, this optimization can help reduce
> the occurrence of direct reclaim, which is more disruptive to application
> performance.

Could you describe the problem you are trying to address in more details
please? Because your patch is significantly changing the behavior of the
low limit. I would even go as far as say it breaks its expecations
because low limit should provide a certain level of protection and
your patch would allow kswapd to reclaim from those cgroups much sooner
now. If this is really needed then we need much more detailed
justification and also evaluation how that influences existing users.

> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  mm/vmscan.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c80fcae7f2a1..231c66fcdfd8 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -7147,6 +7147,13 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int highest_zoneidx)
>  		goto restart;
>  	}
>  
> +	/* Restart if we skipped the memory low event */
> +	if (sc.memcg_low_skipped && !sc.memcg_low_reclaim &&
> +	    sc.priority < 1) {
> +		sc.memcg_low_reclaim = 1;
> +		goto restart;
> +	}
> +
>  	if (!sc.nr_reclaimed)
>  		atomic_inc(&pgdat->kswapd_failures);
>  
> -- 
> 2.43.0

-- 
Michal Hocko
SUSE Labs