linux-kernel - Re: [patch 4 15/22] timer: Remove slack leftovers

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Date:	Sat, 23 Jul 2016 00:54:48 +0200
From:	"Jason A. Donenfeld" <Jason@...c4.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Chris Mason <clm@...com>,
	Arjan van de Ven <arjan@...radead.org>, rt@...utronix.de,
	Rik van Riel <riel@...hat.com>,
	George Spelvin <linux@...encehorizons.net>,
	Len Brown <lenb@...nel.org>,
	Josh Triplett <josh@...htriplett.org>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch 4 15/22] timer: Remove slack leftovers

On Fri, Jul 22, 2016 at 5:18 PM, Jason A. Donenfeld <Jason@...c4.com> wrote:
> Hi Thomas,
>
> On Fri, Jul 22, 2016 at 3:04 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> Well, this really depends on the TIMEOUT value you have. The code now does
>> implicit batching for larger timeouts by queueing the timers into wheels with
>> coarse grained granularity. As long as your new TIMEOUT value ends up in the
>> same bucket then that's equivalent to the slack thing.
>>
>> Can you give me a ballpark of your TIMEOUT value?
>
> Generally either 5 seconds, 10 seconds, or 25 seconds.
>
> Are these okay? Is the 25 case substantially different from the 5 case?

I suppose, anyway, it's easy enough just to provide my own coalescing
function. Actually, I'd rather have it batch timers to fire earlier
rather than later, in my case. So, I can do something like this to
round down to the nearest ~ quarter of a second:

    static inline unsigned long slack_time(unsigned long time)
    {
       return time & ~(BIT_MASK(ilog2(HZ / 4) + 1) - 1);
    }

    mod_timer(&timer, slack_time(jiffies + TIMEOUT));

This seems to do roughly what I want.

Jason