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

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Message-ID: <YmHwjdfZJJ2DeLTK@zx2c4.com>
Date:   Fri, 22 Apr 2022 02:02:21 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Charles-Francois Natali <cf.natali@...il.com>
Cc:     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.

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