netdev - Re: [PATCH] WireGuard: restrict packet handling to non-isolated CPUs.

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Date:   Thu, 21 Apr 2022 17:40:07 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Charles-Francois Natali <cf.natali@...il.com>,
        wireguard@...ts.zx2c4.com, netdev@...r.kernel.org,
        linux-crypto@...r.kernel.org,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH] WireGuard: restrict packet handling to non-isolated
 CPUs.

On Fri, 22 Apr 2022 02:02:21 +0200
"Jason A. Donenfeld" <Jason@...c4.com> wrote:

> netdev@ - Original thread is at
> https://lore.kernel.org/wireguard/20220405212129.2270-1-cf.natali@gmail.com/
> 
> Hi Charles-François,
> 
> On Tue, Apr 05, 2022 at 10:21:29PM +0100, Charles-Francois Natali wrote:
> > WireGuard currently uses round-robin to dispatch the handling of
> > packets, handling them on all online CPUs, including isolated ones
> > (isolcpus).
> > 
> > This is unfortunate because it causes significant latency on isolated
> > CPUs - see e.g. below over 240 usec:
> > 
> > kworker/47:1-2373323 [047] 243644.756405: funcgraph_entry: |
> > process_one_work() { kworker/47:1-2373323 [047] 243644.756406:
> > funcgraph_entry: | wg_packet_decrypt_worker() { [...]
> > kworker/47:1-2373323 [047] 243644.756647: funcgraph_exit: 0.591 us | }
> > kworker/47:1-2373323 [047] 243644.756647: funcgraph_exit: ! 242.655 us
> > | }
> > 
> > Instead, restrict to non-isolated CPUs.  
> 
> Huh, interesting... I haven't seen this feature before. What's the
> intended use case? To never run _anything_ on those cores except
> processes you choose? To run some things but not intensive things? Is it
> sort of a RT-lite?
> 
> I took a look in padata/pcrypt and it doesn't look like they're
> examining the housekeeping mask at all. Grepping for
> housekeeping_cpumask doesn't appear to show many results in things like
> workqueues, but rather in core scheduling stuff. So I'm not quite sure
> what to make of this patch.
> 
> I suspect the thing to do might be to patch both wireguard and padata,
> and send a patch series to me, the padata people, and
> netdev@...r.kernel.org, and we can all hash this out together.
> 
> Regarding your patch, is there a way to make that a bit more succinct,
> without introducing all of those helper functions? It seems awfully
> verbose for something that seems like a matter of replacing the online
> mask with the housekeeping mask.
> 
> Jason

Applications like DPDK that do polling often use isolcpus or cgroups
to keep unwanted rabble off of their cpus.  Having wireguard use those
cpus seems bad.