linux-kernel - Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg

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Date:   Thu, 27 Jul 2017 10:59:50 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     jgross@...e.com, Stefano Stabellini <stefano@...reto.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org
Subject: Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg

On 07/26/2017 08:08 PM, Stefano Stabellini wrote:
> On Wed, 26 Jul 2017, Boris Ostrovsky wrote:
>>>> +			count++;
>>>> +		else
>>>> +			wait_event_interruptible(map->active.inflight_conn_req,
>>>> +						 pvcalls_front_read_todo(map));
>>>> +	}
>>> Should we be using PVCALLS_FRONT_MAX_SPIN here? In sendmsg it is
>>> counting non-sleeping iterations but here we are sleeping so
>>> PVCALLS_FRONT_MAX_SPIN (5000) may take a while.
>>>
>>> In fact, what shouldn't this waiting be a function of MSG_DONTWAIT
>> err, which it already is. But the question still stands (except for
>> MSG_DONTWAIT).
> The code (admittedly unintuitive) is busy-looping (non-sleeping) for
> 5000 iterations *before* attempting to sleep. So in that regard, recvmsg
> and sendmsg use PVCALLS_FRONT_MAX_SPIN in the same way: only for
> non-sleeping iterations.
>

OK.

Why not go directly into wait_event_interruptible()? I see you write in
the commit message

If not enough data is available on the ring, rather than returning
immediately or sleep-waiting, spin for up to 5000 cycles. This small
optimization turns out to improve performance and latency significantly.


Is this because of scheduling latency? I think this should be mentioned not just in the commit message but also as a comment in the code.

(I also think it's not "not enough data" but rather "no data"?)



-boris