linux-kernel - [PATCH v3 0/4] sched_clock: Optimize and avoid deadlock during read from NMI

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Message-Id: <1422644602-11953-1-git-send-email-daniel.thompson@linaro.org>
Date:	Fri, 30 Jan 2015 19:03:18 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>
Cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	linux-kernel@...r.kernel.org, patches@...aro.org,
	linaro-kernel@...ts.linaro.org,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: [PATCH v3 0/4] sched_clock: Optimize and avoid deadlock during read from NMI

This patchset optimizes the generic sched_clock implementation to
significantly reduce the data cache profile. It also makes it safe to call
sched_clock() from NMI (or FIQ on ARM).

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.

v3:
* Optimized to minimise cache profile, including elimination of
  the suspended flag (Thomas Gleixner).
* Replaced the update_bank_begin/end with a single update function
  (Thomas Gleixner).
* Split into multiple patches to aid review.

v2:

* Extended the scope of the read lock in sched_clock() so we can bank
  all data consumed there (John Stultz)

Daniel Thompson (4):
  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: Avoid deadlock during read from NMI

 kernel/time/sched_clock.c | 163 ++++++++++++++++++++++++++++++----------------
 1 file changed, 107 insertions(+), 56 deletions(-)

--
1.9.3

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