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

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Date:	Fri, 22 Jul 2016 13:31:46 +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

Hi Thomas,

Thomas Gleixner <tglx@...utronix.de> writes:
> We now have implicit batching in the timer wheel. The slack is not longer
> used. Remove it.
> - set_timer_slack(&ev->dwork.timer, intv / 4);
> - set_timer_slack(&host->timeout_timer, HZ);
> - set_timer_slack(&di->work.timer, poll_interval * HZ / 4);
> - set_timer_slack(&ohci->io_watchdog, msecs_to_jiffies(20));
> - set_timer_slack(&xhci->comp_mode_recovery_timer,
> etc...

The current mod_timer implementation has this code:

        /*
         * This is a common optimization triggered by the
         * networking code - if the timer is re-modified
         * to be the same thing then just return:
         */
        if (timer_pending(timer) && timer->expires == expires)
                return 1;

In a (currently out-of-tree, but hopefully mergable soon) driver of
mine, I call mod_timer in the hot path. Every time some very frequent
event happens, I call mod_timer to move the timer back to be at
"jiffies + TIMEOUT".

>From a brief look at timer.c, it looked like __mod_timer was rather
expensive. So, as an optimization, I wanted the "timer_pending(timer)
&& timer->expires == expires" condition to be hit in most of the
cases. I accomplished this by doing:

    set_timer_slack(timer, HZ / 4);

This ensured that we'd only wind up calling __mod_timer 4 times per
second, at most.

With the removal of the slack concept, I no longer can do this. I
haven't reviewed this series in depth, but I'm wondering if you'd
recommend a different optimization instead. Or, have things been
reworked so much, that calling mod_timer is now always inexpensive?

Thanks,
Jason