Re: [PATCH net-next 1/2] net: Keep sk->sk_forward_alloc as a proper size

Eric Dumazet
2023-05-17T16:33:40Z

On Wed, May 17, 2023 at 6:24 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Tue, May 16, 2023 at 01:46:55PM +0800, Oliver Sang wrote:
> > hi Shakeel,
> >
> > On Mon, May 15, 2023 at 12:50:31PM -0700, Shakeel Butt wrote:
> > > +Feng, Yin and Oliver
> > >
> > > >
> > > > > Thanks a lot Cathy for testing. Do you see any performance improvement for
> > > > > the memcached benchmark with the patch?
> > > >
> > > > Yep, absolutely :- ) RPS (with/without patch) = +1.74
> > >
> > > Thanks a lot Cathy.
> > >
> > > Feng/Yin/Oliver, can you please test the patch at [1] with other
> > > workloads used by the test robot? Basically I wanted to know if it has
> > > any positive or negative impact on other perf benchmarks.
> >
> > is it possible for you to resend patch with Signed-off-by?
> > without it, test robot will regard the patch as informal, then it cannot feed
> > into auto test process.
> > and could you tell us the base of this patch? it will help us apply it
> > correctly.
> >
> > on the other hand, due to resource restraint, we normally cannot support
> > this type of on-demand test upon a single patch, patch set, or a branch.
> > instead, we try to merge them into so-called hourly-kernels, then distribute
> > tests and auto-bisects to various platforms.
> > after we applying your patch and merging it to hourly-kernels sccussfully,
> > if it really causes some performance changes, the test robot could spot out
> > this patch as 'fbc' and we will send report to you. this could happen within
> > several weeks after applying.
> > but due to the complexity of whole process (also limited resourse, such like
> > we cannot run all tests on all platforms), we cannot guanrantee capture all
> > possible performance impacts of this patch. and it's hard for us to provide
> > a big picture like what's the general performance impact of this patch.
> > this maybe is not exactly what you want. is it ok for you?
> >
> >
>
> Yes, that is fine and thanks for the help. The patch is below:
>
>
> From 93b3b4c5f356a5090551519522cfd5740ae7e774 Mon Sep 17 00:00:00 2001
> From: Shakeel Butt <shakeelb@google.com>
> Date: Tue, 16 May 2023 20:30:26 +0000
> Subject: [PATCH] memcg: skip stock refill in irq context
>
> The linux kernel processes incoming packets in softirq on a given CPU
> and those packets may belong to different jobs. This is very normal on
> large systems running multiple workloads. With memcg enabled, network
> memory for such packets is charged to the corresponding memcgs of the
> jobs.
>
> Memcg charging can be a costly operation and the memcg code implements
> a per-cpu memcg charge caching optimization to reduce the cost of
> charging. More specifically, the kernel charges the given memcg for more
> memory than requested and keep the remaining charge in a local per-cpu
> cache. The insight behind this heuristic is that there will be more
> charge requests for that memcg in near future. This optimization works
> well when a specific job runs on a CPU for long time and majority of the
> charging requests happen in process context. However the kernel's
> incoming packet processing does not work well with this optimization.
>
> Recently Cathy Zhang has shown [1] that memcg charge flushing within the
> memcg charge path can become a performance bottleneck for the memcg
> charging of network traffic.
>
> Perf profile:
>
> 8.98% mc-worker [kernel.vmlinux] [k] page_counter_cancel
> |
> --8.97%--page_counter_cancel
> |
> --8.97%--page_counter_uncharge
> drain_stock
> __refill_stock
> refill_stock
> |
> --8.91%--try_charge_memcg
> mem_cgroup_charge_skmem
> |
> --8.91%--__sk_mem_raise_allocated
> __sk_mem_schedule
> |
> |--5.41%--tcp_try_rmem_schedule
> | tcp_data_queue
> | tcp_rcv_established
> | tcp_v4_do_rcv
> | tcp_v4_rcv
>
> The simplest way to solve this issue is to not refill the memcg charge
> stock in the irq context. Since networking is the main source of memcg
> charging in the irq context, other users will not be impacted. In
> addition, this will preseve the memcg charge cache of the application
> running on that CPU.
>
> There are also potential side effects. What if all the packets belong to
> the same application and memcg? More specifically, users can use Receive
> Flow Steering (RFS) to make sure the kernel process the packets of the
> application on the CPU where the application is running. This change may
> cause the kernel to do slowpath memcg charging more often in irq
> context.

Could we have per-memcg per-cpu caches, instead of one set of per-cpu caches
needing to be drained evertime a cpu deals with 'another memcg' ?

>
> Link: https://lore.kernel.org/all/IA0PR11MB73557DEAB912737FD61D2873FC749@IA0PR11MB7355.namprd11.prod.outlook.com [1]
> Signed-off-by: Shakeel Butt <shakeelb@google.com>
> ---
> mm/memcontrol.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 5abffe6f8389..2635aae82b3e 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2652,6 +2652,14 @@ static int try_charge_memcg(struct mem_cgroup *memcg, gfp_t gfp_mask,
> bool raised_max_event = false;
> unsigned long pflags;
>
> + /*
> + * Skip the refill in irq context as it may flush the charge cache of
> + * the process running on the CPUs or the kernel may have to process
> + * incoming packets for different memcgs.
> + */
> + if (!in_task())
> + batch = nr_pages;
> +
> retry:
> if (consume_stock(memcg, nr_pages))
> return 0;
> --
> 2.40.1.606.ga4b1b128d6-goog
>