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

[Peter Zijlstra <peterz@infradead.org>]

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.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>