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 <[email protected]>
Acme, if you agree with the tools part, feel free to take the lot. Acked-by: Peter Zijlstra (Intel) <[email protected]> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
