linux-kernel - Re: [PATCH net-next] net: rename low latency sockets functions to busy poll

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Date:	Mon, 08 Jul 2013 22:46:04 +0300
From:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	David Miller <davem@...emloft.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH net-next] net: rename low latency sockets functions to
 busy poll

On 08/07/2013 22:37, Linus Torvalds wrote:
> On Mon, Jul 8, 2013 at 10:14 AM, Eliezer Tamir
> <eliezer.tamir@...ux.intel.com> wrote:
>>
>> I think there is no way for the compiler to know the value of
>> can_busy_loop at compile time. It depends on the replies we get
>> from polling the sockets. ll_flag was there to make sure the compiler
>> will know when things are defined out.
> 
> No, my point was that we want to handle the easily seen register test
> first, and not even have to load current().
> 
> The compiler may end up scheduling the code to load current anyway,
> but the way you wrote it it's pretty much guaranteed that it will do
> it.

I see. OK.

> In fact, I'd argue for initializing start_time to zero, and have the
> "have we timed out" logic load it only if necessary, rather than
> initializing it based on whether POLL_BUSY_WAIT was set or not.
> Because one common case - even with POLL_BUSY_WAIT - is that we go
> through the loop exactly once, and the data exists on the very first
> try. And that is in fact the case we want to optimize and not do any
> extra work for at all.
> 
> So I would actually argue that the whole timeout code might as well be
> something like
> 
>     unsigned long start_time = 0;
>     ...
>     if (want_busy_poll && !need_resched()) {
>         unsigned long now = busy_poll_sched_clock();
>         if (!start_time) {
>             start_time = now + sysctl.busypoll;
>             continue;
>         }
>         if (time_before(start_time, now))
>             continue;
>     }
> 

OK.

Thanks,
Eliezer
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