From: "Toke Høiland-Jørgensen" <toke@toke.dk>
To: "Jason A. Donenfeld" <Jason@zx2c4.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: WireGuard mailing list <wireguard@lists.zx2c4.com>,
Netdev <netdev@vger.kernel.org>,
"David S. Miller" <davem@davemloft.net>,
Jakub Kicinski <kuba@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: [RFC] wiregard RX packet processing.
Date: Wed, 05 Jan 2022 01:14:47 +0100 [thread overview]
Message-ID: <87mtkbavig.fsf@toke.dk> (raw)
In-Reply-To: <CAHmME9rzEjKg41eq5jBtsLXF+vZSEnvdomZJ-rTzx8Q=ac1ayg@mail.gmail.com>
"Jason A. Donenfeld" <Jason@zx2c4.com> writes:
> Hi Sebastian,
>
> Seems like you've identified two things, the use of need_resched, and
> potentially surrounding napi_schedule in local_bh_{disable,enable}.
>
> Regarding need_resched, I pulled that out of other code that seemed to
> have the "same requirements", as vaguely conceived. It indeed might
> not be right. The intent is to have that worker running at maximum
> throughput for extended periods of time, but not preventing other
> threads from running elsewhere, so that, e.g., a user's machine
> doesn't have a jenky mouse when downloading a file.
>
> What are the effects of unconditionally calling cond_resched() without
> checking for if (need_resched())? Sounds like you're saying none at
> all?
I believe so: AFAIU, you use need_resched() if you need to do some kind
of teardown before the schedule point, like this example I was recently
looking at:
https://elixir.bootlin.com/linux/latest/source/net/bpf/test_run.c#L73
If you just need to maybe reschedule, you can just call cond_resched()
and it'll do what it says on the tin: do a schedule if needed, and
return immediately otherwise.
> Regarding napi_schedule, I actually wasn't aware that it's requirement
> to _only_ ever run from softirq was a strict one. When I switched to
> using napi_schedule in this way, throughput really jumped up
> significantly. Part of this indeed is from the batching, so that the
> napi callback can then handle more packets in one go later. But I
> assumed it was something inside of NAPI that was batching and
> scheduling it, rather than a mistake on my part to call this from a wq
> and not from a softirq.
>
> What, then, are the effects of surrounding that in
> local_bh_{disable,enable} as you've done in the patch? You mentioned
> one aspect is that it will "invoke wg_packet_rx_poll() where you see
> only one skb." It sounds like that'd be bad for performance, though,
> given that the design of napi is really geared toward batching.
Heh, I wrote a whole long explanation he about variable batch sizes
because you don't control when the NAPI is scheduled, etc... And then I
noticed the while loop is calling ptr_ring_consume_bh(), which means
that there's already a local_bh_disable/enable pair on every loop
invocation. So you already have this :)
Which of course raises the question of whether there's anything to gain
from *adding* batching to the worker? Something like:
#define BATCH_SIZE 8
void wg_packet_decrypt_worker(struct work_struct *work)
{
struct crypt_queue *queue = container_of(work, struct multicore_worker,
work)->ptr;
void *skbs[BATCH_SIZE];
bool again;
int i;
restart:
local_bh_disable();
ptr_ring_consume_batched(&queue->ring, skbs, BATCH_SIZE);
for (i = 0; i < BATCH_SIZE; i++) {
struct sk_buff *skb = skbs[i];
enum packet_state state;
if (!skb)
break;
state = likely(decrypt_packet(skb, PACKET_CB(skb)->keypair)) ?
PACKET_STATE_CRYPTED : PACKET_STATE_DEAD;
wg_queue_enqueue_per_peer_rx(skb, state);
}
again = !ptr_ring_empty(&queue->ring);
local_bh_enable();
if (again) {
cond_resched();
goto restart;
}
}
Another thing that might be worth looking into is whether it makes sense
to enable threaded NAPI for Wireguard. See:
https://lore.kernel.org/r/20210208193410.3859094-1-weiwan@google.com
-Toke
next prev parent reply other threads:[~2022-01-05 0:17 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-08 17:32 Sebastian Andrzej Siewior
2021-12-20 17:29 ` Jason A. Donenfeld
2022-01-05 0:14 ` Toke Høiland-Jørgensen [this message]
2022-01-11 15:40 ` Sebastian Andrzej Siewior
2022-02-10 0:21 ` Feature Request :: Configuration-Option "ospf = yes|no" on Multi-Peer-Interfaces markus
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