Re: [PATCH net] net-memcg: avoid stalls when under memory pressure

[Roman Gushchin <roman.gushchin@linux.dev>]

On Fri, Oct 21, 2022 at 09:03:04AM -0700, Jakub Kicinski 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@google.com>
> Fixes: 4b1327be9fe5 ("net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem()")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Acked-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!