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Date:   Fri, 21 Oct 2022 09:25:52 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     edumazet@...gle.com, netdev@...r.kernel.org, davem@...emloft.net,
        pabeni@...hat.com, cgroups@...r.kernel.org,
        roman.gushchin@...ux.dev, weiwan@...gle.com, ncardwell@...gle.com,
        ycheng@...gle.com
Subject: Re: [PATCH net] net-memcg: avoid stalls when under memory pressure

On Fri, Oct 21, 2022 at 9:03 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> As Shakeel explains the commit under Fixes had the unintended
> side-effect of no longer pre-loading the cached memory allowance.
> Even tho we previously dropped the first packet received when
> over memory limit - the consecutive ones would get thru by using
> the cache. The charging was happening in batches of 128kB, so
> we'd let in 128kB (truesize) worth of packets per one drop.
>
> After the change we no longer force charge, there will be no
> cache filling side effects. This causes significant drops and
> connection stalls for workloads which use a lot of page cache,
> since we can't reclaim page cache under GFP_NOWAIT.
>
> Some of the latency can be recovered by improving SACK reneg
> handling but nowhere near enough to get back to the pre-5.15
> performance (the application I'm experimenting with still
> sees 5-10x worst latency).
>
> Apply the suggested workaround of using GFP_ATOMIC. We will now
> be more permissive than previously as we'll drop _no_ packets
> in softirq when under pressure. But I can't think of any good
> and simple way to address that within networking.
>
> Link: https://lore.kernel.org/all/20221012163300.795e7b86@kernel.org/
> Suggested-by: Shakeel Butt <shakeelb@...gle.com>
> Fixes: 4b1327be9fe5 ("net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem()")
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
> CC: weiwan@...gle.com
> CC: shakeelb@...gle.com
> CC: ncardwell@...gle.com
> CC: ycheng@...gle.com
> ---
>  include/net/sock.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 9e464f6409a7..22f8bab583dd 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -2585,7 +2585,7 @@ static inline gfp_t gfp_any(void)
>
>  static inline gfp_t gfp_memcg_charge(void)
>  {
> -       return in_softirq() ? GFP_NOWAIT : GFP_KERNEL;
> +       return in_softirq() ? GFP_ATOMIC : GFP_KERNEL;
>  }
>

How about just using gfp_any() and we can remove gfp_memcg_charge()?

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