netdev - Re: [PATCH net-next v6 08/12] net: homa: create homa

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Message-ID: <82cdba95-83cb-4902-bb2a-a2ab880797a8@redhat.com>
Date: Thu, 30 Jan 2025 10:57:30 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: John Ousterhout <ouster@...stanford.edu>
Cc: netdev@...r.kernel.org, edumazet@...gle.com, horms@...nel.org,
 kuba@...nel.org
Subject: Re: [PATCH net-next v6 08/12] net: homa: create homa_incoming.c



On 1/30/25 1:48 AM, John Ousterhout wrote:
> On Mon, Jan 27, 2025 at 2:19 AM Paolo Abeni <pabeni@...hat.com> wrote:
>>
>> On 1/15/25 7:59 PM, John Ousterhout wrote:
>>> +     /* Each iteration through the following loop processes one packet. */
>>> +     for (; skb; skb = next) {
>>> +             h = (struct homa_data_hdr *)skb->data;
>>> +             next = skb->next;
>>> +
>>> +             /* Relinquish the RPC lock temporarily if it's needed
>>> +              * elsewhere.
>>> +              */
>>> +             if (rpc) {
>>> +                     int flags = atomic_read(&rpc->flags);
>>> +
>>> +                     if (flags & APP_NEEDS_LOCK) {
>>> +                             homa_rpc_unlock(rpc);
>>> +                             homa_spin(200);
>>
>> Why spinning on the current CPU here? This is completely unexpected, and
>> usually tolerated only to deal with H/W imposed delay while programming
>> some device registers.
> 
> This is done to pass the RPC lock off to another thread (the
> application); the spin is there to allow the other thread to acquire
> the lock before this thread tries to acquire it again (almost
> immediately). There's no performance impact from the spin because this
> thread is going to turn around and try to acquire the RPC lock again
> (at which point it will spin until the other thread releases the
> lock). Thus it's either spin here or spin there. I've added a comment
> to explain this.

What if another process is spinning on the RPC lock without setting
APP_NEEDS_LOCK? AFAICS incoming packets targeting the same RPC could
land on different RX queues.

If the spin is not functionally needed, just drop it. If it's needed, it
would be better to find some functional replacement, possibly explicit
notification via waitqueue or completion.

/P