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.hun...@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) <pet...@infradead.org> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
