[PATCH 0/5] Arch-generic sched_clock NMI safety and optimizations for -tip

John Stultz
2015-03-17T17:38:33Z

Ingo, Peter, Thomas,

The following patches from Daniel extend the arch-generic sched_clock
implementation so that it can be safely called from NMI (or FIQ on ARM)
context. They also optimize the sched_clock logic to improve cache
performance. I wanted to send them along so they could be queued in
-tip for 4.1.

>From Daniel's last post:
"The data cache profile of sched_clock() in both the original code and
my previous patch was somewhere between 2 and 3 (64-byte) cache lines,
depending on alignment of struct clock_data. After patching, the cache
profile for the normal case should be a single cacheline.

NMI safety was tested on i.MX6 with perf drowning the system in FIQs and
using the perf handler to check that sched_clock() returned monotonic
values. At the same time I forcefully reduced kt_wrap so that
update_sched_clock() is being called at >1000Hz.

Without the patches the above system is grossly unstable, surviving
[9K,115K,25K] perf event cycles during three separate runs. With the
patch I ran for over 9M perf event cycles before getting bored."

I'm relaying these along because in the past I've queued and submitted
arch-generic sched_clock.c changes from Stephen via Thomas, but I wanted
to make sure this got a look from Ingo and Peter.

In the future, I think Stephen is probably the right person to act as
patch-catcher for the kernel/time/sched_clock.c file. But I think those
changes should continue to be merged in via the -tip tree w/ other
scheduler code.

Let me know if you have any objections or concerns.

thanks
-john

Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>

These patches can also be pulled from the pull request below.

The following changes since commit c517d838eb7d07bbe9507871fab3931deccff539:

Linux 4.0-rc1 (2015-02-22 18:21:14 -0800)

are available in the git repository at:

https://git.linaro.org/people/john.stultz/linux.git fortglx/4.1/schedclock

for you to fetch changes up to 4bb74b85d2e74cfec9e21a6de44b232a2983bcc7:

sched_clock: Avoid deadlock during read from NMI (2015-03-10 20:28:58 -0700)

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

Daniel Thompson (5):
sched_clock: Match scope of read and write seqcounts
sched_clock: Optimize cache line usage
sched_clock: Remove suspend from clock_read_data
sched_clock: Remove redundant notrace from update function
sched_clock: Avoid deadlock during read from NMI

kernel/time/sched_clock.c | 195 ++++++++++++++++++++++++++++++++--------------
1 file changed, 138 insertions(+), 57 deletions(-)

--
1.9.1