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).
[ v2: address review comments by Andrew Morton. ]
Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@linux-foundation.org>
Orig-LKML-Reference: <18890.6578.728637.139402@cargo.ozlabs.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
53cfbf5937