linux-kernel - [PATCH v3 tip/core/timers 0/5] Crude timer-wheel latency hacks

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Message-ID: <20140116040203.GA5206@linux.vnet.ibm.com>
Date:	Wed, 15 Jan 2014 20:02:03 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	oleg@...hat.com, sbw@....edu
Subject: [PATCH v3 tip/core/timers 0/5] Crude timer-wheel latency hacks

Hello!

The following five patches provide some crude timer-wheel latency patches.
I understand that a more comprehensive solution is in progress, but in the
meantime, these patches work well in cases where a given CPU has either
zero or one timers pending, which is a common case for NO_HZ_FULL kernels.
Note that these patches do not help in the case where a given timer wheel
has a pair of widely separated timers, while the more comprehensive
solution is likely to handle more gracefully.  So, on the off-chance
that this is helpful to someone, the individual patches are as follows:

1.	Add ->all_timers field to tbase_vec to count all timers, not
	just the non-deferrable ones.

2.	Avoid jiffy-at-a-time stepping when the timer wheel is empty.

3.	Avoid jiffy-at-a-time stepping when the timer wheel transitions
	to empty.

4.	Avoid jiffy-at-a-time stepping after a timer is added to an
	initially empty timer wheel.

5.	Make internal_add_timer() update ->next_timer if ->active_timers == 0,
	courtesy of Oleg Nesterov.

Differences from v2:

o	Add patch #5 from Oleg.

o	Move the check for adding to an empty timer wheel from mod_timer()
	to internal_add_timer(), as suggested by Oleg Nesterov.

o	Addressed review comments from Oleg, Steven Rostedt, Josh Triplett,
	and Fengguang Wu.

Differences from v1:

o	Fix an embarrassing bug located by Oleg Nesterov where the
	timer wheel could be judged to be empty even if it contained
	deferrable timers.

							Thanx, Paul

------------------------------------------------------------------------

 b/kernel/timer.c |   30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

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