Re: [PATCH V4 1/5] perf: Add PERF_RECORD_SWITCH to indicate context switches

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Tue, Jul 21, 2015 at 12:29:55PM +0200, Peter Zijlstra escreveu:
> On Tue, Jul 21, 2015 at 12:44:02PM +0300, Adrian Hunter wrote:
> > There are already two events for context switches, namely
> > the tracepoint sched:sched_switch and the software event
> > context_switches. Unfortunately neither are suitable for
> > use by non-privileged users for the purpose of synchronizing
> > hardware trace data (e.g. Intel PT) to the context switch.
> > 
> > Tracepoints are no good at all for non-privileged users
> > because they need either CAP_SYS_ADMIN or
> > /proc/sys/kernel/perf_event_paranoid <= -1.
> > 
> > On the other hand, kernel software events need either
> > CAP_SYS_ADMIN or /proc/sys/kernel/perf_event_paranoid <= 1.
> > 
> > Now many distributions do default perf_event_paranoid to 1
> > making context_switches a contender, except it has another
> > problem (which is also shared with sched:sched_switch)
> > which is that it happens before perf schedules events out
> > instead of after perf schedules events in. Whereas a
> > privileged user can see all the events anyway, a
> > non-privileged user only sees events for their own processes,
> > in other words they see when their process was scheduled out
> > not when it was scheduled in. That presents two problems to
> > use the event: 1. the information comes too late, so tools
> > have to look ahead in the event stream to find out what the
> > current state is 2. if they are unlucky tracing might have
> > stopped before the context-switches event is recorded.
> > 
> > This new PERF_RECORD_SWITCH event does not have those problems
> > and it also has a couple of other small advantages. It is
> > easier to use because it is an auxiliary event (like mmap,
> > comm and task events) which can be enabled by setting a single
> > bit. It is smaller than sched:sched_switch and easier to parse.
> > 
> > To make the event useful for privileged users also, if the
> > context is cpu-wide then the event record will be
> > PERF_RECORD_SWITCH_CPU_WIDE which is the same as
> > PERF_RECORD_SWITCH except it also provides the next or
> > previous pid/tid.
> > 
> > Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> 
> Acme, if you agree with the tools part, feel free to take the lot.

Looking at it, if all is well, I'll do it
 
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>