On Thu, Dec 06, 2018 at 01:19:46PM -0500, Steven Rostedt wrote: > On Thu, 6 Dec 2018 09:34:00 +0100 > Peter Zijlstra <pet...@infradead.org> wrote: > > > > > > > I don't understand this.. why are we using schedule_timeout() and all > > > that? > > > > Urgh.. in fact, the more I look at this the more I hate it. > > > > We want to block in __perf_output_begin(), but we cannot because both > > tracepoints and perf will have preemptability disabled down there. > > > > So what we do is fail the event, fake the lost count and go all the way > > up that callstack, detect the failure and then poll-wait and retry. > > > > And only do this for a few special events... *yuck* > > Since this is a special case, we should add a new option to the perf > system call that, 1 states that it wants the traced process to block > (and must have PTRACE permission to do so) and 2, after it reads from > the buffer, it needs to check a bit that says "this process is blocked, > please wake it up" and then do another perf call to kick the process to > continue.
so instead of polling the traced process would properly wait for tracer to kick him again after it reads/frees the buffer I guess we could use the control mmap page (struct perf_event_mmap_page) to communicate the 'we are block-ed' message to the tracer and have new ioctl to wake the waiting process jirka > > I really dislike the polling too. But because this is not a default > case, and is a new feature, we can add more infrastructure to make it > work properly, instead of trying to hack the current method into > something that does something poorly. > > -- Steve
