Subject: Re: [PATCH v3] perfcounters: record time running and time enabled
 for each counter
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@linux-foundation.org>
In-Reply-To: <18890.6578.728637.139402@cargo.ozlabs.ibm.com>
References: <18890.6578.728637.139402@cargo.ozlabs.ibm.com>
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Date: Wed, 25 Mar 2009 13:21:08 +0100
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On Wed, 2009-03-25 at 22:46 +1100, Paul Mackerras wrote:
> Impact: new functionality
> 
> Currently, if there are more counters enabled than can fit on the CPU,
> the kernel will multiplex the counters on to the hardware using
> round-robin scheduling.  That isn't too bad for sampling counters, but
> for counting counters it means that the value read from a counter
> represents some unknown fraction of the true count of events that
> occurred while the counter was enabled.
> 
> This remedies the situation by keeping track of how long each counter
> is enabled for, and how long it is actually on the cpu and counting
> events.  These times are recorded in nanoseconds using the task clock
> for per-task counters and the cpu clock for per-cpu counters.
> 
> These values can be supplied to userspace on a read from the counter.
> Userspace requests that they be supplied after the counter value by
> setting the PERF_FORMAT_TOTAL_TIME_ENABLED and/or
> PERF_FORMAT_TOTAL_TIME_RUNNING bits in the hw_event.read_format field
> when creating the counter.  (There is no way to change the read format
> after the counter is created, though it would be possible to add some
> way to do that.)
> 
> Using this information it is possible for userspace to scale the count
> it reads from the counter to get an estimate of the true count:
> 
> true_count_estimate = count * total_time_enabled / total_time_running
> 
> This also lets userspace detect the situation where the counter never
> got to go on the cpu: total_time_running == 0.
> 
> This functionality has been requested by the PAPI developers, and will
> be generally needed for interpreting the count values from counting
> counters correctly.
> 
> In the implementation, this keeps 5 time values (in nanoseconds) for
> each counter: total_time_enabled and total_time_running are used when
> the counter is in state OFF or ERROR and for reporting back to
> userspace.  When the counter is in state INACTIVE or ACTIVE, it is the
> tstamp_enabled, tstamp_running and tstamp_stopped values that are
> relevant, and total_time_enabled and total_time_running are determined
> from them.  (tstamp_stopped is only used in INACTIVE state.)  The
> reason for doing it like this is that it means that only counters
> being enabled or disabled at sched-in and sched-out time need to be
> updated.  There are no new loops that iterate over all counters to
> update total_time_enabled or total_time_running.
> 
> This also keeps separate child_total_time_running and
> child_total_time_enabled fields that get added in when reporting the
> totals to userspace.  They are separate fields so that they can be
> atomic.  We don't want to use atomics for total_time_running,
> total_time_enabled etc., because then we would have to use atomic
> sequences to update them, which are slower than regular arithmetic and
> memory accesses.
> 
> It is possible to measure total_time_running by adding a task_clock
> counter to each group of counters, and total_time_enabled can be
> measured approximately with a top-level task_clock counter (though
> inaccuracies will creep in if you need to disable and enable groups
> since it is not possible in general to disable/enable the top-level
> task_clock counter simultaneously with another group).  However, that
> adds extra overhead - I measured around 15% increase in the context
> switch latency reported by lat_ctx (from lmbench) when a task_clock
> counter was added to each of 2 groups, and around 25% increase when a
> task_clock counter was added to each of 4 groups.  (In both cases a
> top-level task-clock counter was also added.)
> 
> In contrast, the code added in this commit gives better information
> with no overhead that I could measure (in fact in some cases I
> measured lower times with this code, but the differences were all less
> than one standard deviation).
> 
> Signed-off-by: Paul Mackerras <paulus@samba.org>

Looks good,

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

Paul, should we perhaps also put a format header in the sys_read()
output?

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