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From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Sat, 23 Jul 2016 00:54:48 +0200
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Subject: Re: [patch 4 15/22] timer: Remove slack leftovers
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Chris Mason <clm@fb.com>,
        Arjan van de Ven <arjan@infradead.org>, rt@linutronix.de,
        Rik van Riel <riel@redhat.com>,
        George Spelvin <linux@sciencehorizons.net>,
        Len Brown <lenb@kernel.org>, Josh Triplett <josh@joshtriplett.org>,
        Eric Dumazet <edumazet@google.com>
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On Fri, Jul 22, 2016 at 5:18 PM, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> Hi Thomas,
>
> On Fri, Jul 22, 2016 at 3:04 PM, Thomas Gleixner <tglx@linutronix.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