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author | Andi Kleen <ak@linux.intel.com> | 2014-09-02 11:44:14 -0700 |
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committer | Ingo Molnar <mingo@kernel.org> | 2014-09-24 14:48:19 +0200 |
commit | c46e665f037743bbce7887e7c55750713eb7fb09 (patch) | |
tree | b1a83558436d1f82e18f0bf9bd9d5c7ded33e32b /arch/x86/kernel/cpu/perf_event.h | |
parent | 86a349a28b249bf4c3b396af3cc550760dedb456 (diff) | |
download | op-kernel-dev-c46e665f037743bbce7887e7c55750713eb7fb09.zip op-kernel-dev-c46e665f037743bbce7887e7c55750713eb7fb09.tar.gz |
perf/x86: Add INST_RETIRED.ALL workarounds
On Broadwell INST_RETIRED.ALL cannot be used with any period
that doesn't have the lowest 6 bits cleared. And the period
should not be smaller than 128.
Add a new callback to enforce this, and set it for Broadwell.
This is erratum BDM57 and BDM11.
How does this handle the case when an app requests a specific
period with some of the bottom bits set
The apps thinks it is sampling at X occurences per sample, when it is
in fact at X - 63 (worst case).
Short answer:
Any useful instruction sampling period needs to be 4-6 orders
of magnitude larger than 128, as an PMI every 128 instructions
would instantly overwhelm the system and be throttled.
So the +-64 error from this is really small compared to the
period, much smaller than normal system jitter.
Long answer:
<write up by Peter:>
IFF we guarantee perf_event_attr::sample_period >= 128.
Suppose we start out with sample_period=192; then we'll set period_left
to 192, we'll end up with left = 128 (we truncate the lower bits). We
get an interrupt, find that period_left = 64 (>0 so we return 0 and
don't get an overflow handler), up that to 128. Then we trigger again,
at n=256. Then we find period_left = -64 (<=0 so we return 1 and do get
an overflow). We increment with sample_period so we get left = 128. We
fire again, at n=384, period_left = 0 (<=0 so we return 1 and get an
overflow). And on and on.
So while the individual interrupts are 'wrong' we get then with
interval=256,128 in exactly the right ratio to average out at 192. And
this works for everything >=128.
So the num_samples*fixed_period thing is still entirely correct +- 127,
which is good enough I'd say, as you already have that error anyhow.
So no need to 'fix' the tools, al we need to do is refuse to create
INST_RETIRED:ALL events with sample_period < 128.
Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Maria Dimakopoulou <maria.n.dimakopoulou@gmail.com>
Cc: Mark Davies <junk@eslaf.co.uk>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1409683455-29168-4-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/kernel/cpu/perf_event.h')
-rw-r--r-- | arch/x86/kernel/cpu/perf_event.h | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/arch/x86/kernel/cpu/perf_event.h b/arch/x86/kernel/cpu/perf_event.h index fc5eb39..d98a34d 100644 --- a/arch/x86/kernel/cpu/perf_event.h +++ b/arch/x86/kernel/cpu/perf_event.h @@ -445,6 +445,7 @@ struct x86_pmu { struct x86_pmu_quirk *quirks; int perfctr_second_write; bool late_ack; + unsigned (*limit_period)(struct perf_event *event, unsigned l); /* * sysfs attrs |