Peter Zijlstra 09f5e7dc7a perf: Fix perf_event_read_local() time ... Time readers that cannot take locks (due to NMI etc..) currently make use of perf_event::shadow_ctx_time, which, for that event gives: time' = now + (time - timestamp) or, alternatively arranged: time' = time + (now - timestamp) IOW, the progression of time since the last time the shadow_ctx_time was updated. There's problems with this: A) the shadow_ctx_time is per-event, even though the ctx_time it reflects is obviously per context. The direct concequence of this is that the context needs to iterate all events all the time to keep the shadow_ctx_time in sync. B) even with the prior point, the context itself might not be active meaning its time should not advance to begin with. C) shadow_ctx_time isn't consistently updated when ctx_time is There are 3 users of this stuff, that suffer differently from this: - calc_timer_values() - perf_output_read() - perf_event_update_userpage() /* A */ - perf_event_read_local() /* A,B */ In particular, perf_output_read() doesn't suffer at all, because it's sample driven and hence only relevant when the event is actually running. This same was supposed to be true for perf_event_update_userpage(), after all self-monitoring implies the context is active *HOWEVER*, as per commit f792565326 ("perf/core: fix userpage->time_enabled of inactive events") this goes wrong when combined with counter overcommit, in that case those events that do not get scheduled when the context becomes active (task events typically) miss out on the EVENT_TIME update and ENABLED time is inflated (for a little while) with the time the context was inactive. Once the event gets rotated in, this gets corrected, leading to a non-monotonic timeflow. perf_event_read_local() made things even worse, it can request time at any point, suffering all the problems perf_event_update_userpage() does and more. Because while perf_event_update_userpage() is limited by the context being active, perf_event_read_local() users have no such constraint. Therefore, completely overhaul things and do away with perf_event::shadow_ctx_time. Instead have regular context time updates keep track of this offset directly and provide perf_event_time_now() to complement perf_event_time(). perf_event_time_now() will, in adition to being context wide, also take into account if the context is active. For inactive context, it will not advance time. This latter property means the cgroup perf_cgroup_info context needs to grow addition state to track this. Additionally, since all this is strictly per-cpu, we can use barrier() to order context activity vs context time. Fixes: 7d9285e82d ("perf/bpf: Extend the perf_event_read_local() interface, a.k.a. "bpf: perf event change needed for subsequent bpf helpers"") Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Tested-by: Song Liu <song@kernel.org> Tested-by: Namhyung Kim <namhyung@kernel.org> Link: https://lkml.kernel.org/r/YcB06DasOBtU0b00@hirez.programming.kicks-ass.net		2022-01-18 12:09:47 +01:00
..
callchain.c	uaccess: add force_uaccess_{begin,end} helpers	2020-08-12 10:57:59 -07:00
core.c	perf: Fix perf_event_read_local() time	2022-01-18 12:09:47 +01:00
hw_breakpoint.c	perf/hw_breakpoint: Replace deprecated CPU-hotplug functions	2021-08-26 09:14:36 +02:00
internal.h	tracing/perf: Add interrupt_context_level() helper	2021-10-19 20:33:20 -04:00
Makefile	perf/core: allow ftrace for functions in kernel/event/core.c	2021-10-25 22:50:42 -04:00
ring_buffer.c	perf: Cap allocation order at aux_watermark	2021-04-16 16:32:39 +02:00
uprobes.c	mm/memcg: Convert mem_cgroup_charge() to take a folio	2021-09-27 09:27:31 -04:00