netdev - Re: [PATCH] fs/epoll: Enable non-blocking busypoll with epoll timeout of 0

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Date:   Wed, 24 Jun 2020 11:25:43 -0700
From:   "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, netdev@...r.kernel.org,
        "davem@...emloft.net" <davem@...emloft.net>
Subject: Re: [PATCH] fs/epoll: Enable non-blocking busypoll with epoll timeout
 of 0



On 6/24/2020 9:49 AM, Eric Dumazet wrote:
> 
> 
> On 6/24/20 9:32 AM, Samudrala, Sridhar wrote:
>> Adding Dave, Eric for review and see if we can get this in via net-next
>> as this is mainly useful for networking workloads doing busypoll.
>>
>> Thanks
>> Sridhar
>>
>> On 6/19/2020 11:13 AM, Sridhar Samudrala wrote:
>>> This patch triggers non-blocking busy poll when busy_poll is enabled and
>>> epoll is called with a timeout of 0 and is associated with a napi_id.
>>> This enables an app thread to go through napi poll routine once by calling
>>> epoll with a 0 timeout.
>>>
>>> poll/select with a 0 timeout behave in a similar manner.
>>>
>>> Signed-off-by: Sridhar Samudrala <sridhar.samudrala@...el.com>
>>> ---
>>>    fs/eventpoll.c | 13 +++++++++++++
>>>    1 file changed, 13 insertions(+)
>>>
>>> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
>>> index 12eebcdea9c8..5f55078d6381 100644
>>> --- a/fs/eventpoll.c
>>> +++ b/fs/eventpoll.c
>>> @@ -1847,6 +1847,19 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
>>>            eavail = ep_events_available(ep);
>>>            write_unlock_irq(&ep->lock);
>>>    +        /*
>>> +         * Trigger non-blocking busy poll if timeout is 0 and there are
>>> +         * no events available. Passing timed_out(1) to ep_busy_loop
>>> +         * will make sure that busy polling is triggered only once and
>>> +         * only if sysctl.net.core.busy_poll is set to non-zero value.
>>> +         */
>>> +        if (!eavail) {
> 
> Maybe avoid all this stuff for the typical case of busy poll being not used ?
> 
>              if (!evail && net_busy_loop_on)) {

Sure. will submit a v2 with this change.


> 
>>> +            ep_busy_loop(ep, timed_out);
> 
> 
>>> +            write_lock_irq(&ep->lock);
>>> +            eavail = ep_events_available(ep);
>>> +            write_unlock_irq(&ep->lock);
>>> +        }
>>> +
>>>            goto send_events;
>>>        }
>>>