lkml.org 
[lkml]   [2018]   [Feb]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH v1 2/2] perf/core: Add support for PMUs that can be read from any CPU
    On 2018-02-24 00:41, Peter Zijlstra wrote:
    > On Fri, Feb 23, 2018 at 04:19:38PM -0800, Saravana Kannan wrote:
    >> Some PMUs events can be read from any CPU. So allow the PMU to mark
    >> events as such. For these events, we don't need to reject reads or
    >> make smp calls to the event's CPU and cause unnecessary wake ups.
    >>
    >> Good examples of such events would be events from caches shared across
    >> all CPUs.
    >
    > So why would the existing ACTIVE_PKG not work for you? Because clearly
    > your example does not cross a package.

    Because based on testing it on hardware, it looks like the two clusters
    in an ARM DynamIQ design are not considered part of the same "package".
    When I say clusters, I using the more common interpretation of
    "homogeneous CPUs running on the same clock"/CPUs in a cpufreq policy
    and not ARM's new redefinition of cluster. So, on a SoC with 4 little
    and 4 big cores, it'll still trigger a lot of unnecessary smp calls/IPIs
    that cause unnecessary wakeups.

    Although, I like Mark's suggestion of just giving a cpumask for every
    event and using that instead. Because the meaning of "active package" is
    very ambiguous. For example if a SoC has 2 DynamIQ blocks (not sure if
    that's possible), what's considered a package? CPUs that are sitting on
    one L3 can't read the PMU counters of a different L3. In that case,
    neither "Any CPU" nor "Active Package" is correct/usable for reducing
    IPIs.

    >
    >> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
    >> ---
    >> include/linux/perf_event.h | 3 +++
    >> kernel/events/core.c | 10 ++++++++--
    >> 2 files changed, 11 insertions(+), 2 deletions(-)
    >>
    >> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    >> index 7546822..ee8978f 100644
    >> --- a/include/linux/perf_event.h
    >> +++ b/include/linux/perf_event.h
    >> @@ -510,9 +510,12 @@ typedef void (*perf_overflow_handler_t)(struct
    >> perf_event *,
    >> * PERF_EV_CAP_SOFTWARE: Is a software event.
    >> * PERF_EV_CAP_READ_ACTIVE_PKG: A CPU event (or cgroup event) that
    >> can be read
    >> * from any CPU in the package where it is active.
    >> + * PERF_EV_CAP_READ_ANY_CPU: A CPU event (or cgroup event) that can
    >> be read
    >> + * from any CPU.
    >> */
    >> #define PERF_EV_CAP_SOFTWARE BIT(0)
    >> #define PERF_EV_CAP_READ_ACTIVE_PKG BIT(1)
    >> +#define PERF_EV_CAP_READ_ANY_CPU BIT(2)
    >>
    >> #define SWEVENT_HLIST_BITS 8
    >> #define SWEVENT_HLIST_SIZE (1 << SWEVENT_HLIST_BITS)
    >> diff --git a/kernel/events/core.c b/kernel/events/core.c
    >> index 5d3df58..570187b 100644
    >> --- a/kernel/events/core.c
    >> +++ b/kernel/events/core.c
    >> @@ -3484,6 +3484,10 @@ static int __perf_event_read_cpu(struct
    >> perf_event *event, int event_cpu)
    >> {
    >> u16 local_pkg, event_pkg;
    >>
    >> + if (event->group_caps & PERF_EV_CAP_READ_ANY_CPU) {
    >> + return smp_processor_id();
    >> + }
    >> +
    >> if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
    >> int local_cpu = smp_processor_id();
    >>
    >
    >> @@ -3575,6 +3579,7 @@ int perf_event_read_local(struct perf_event
    >> *event, u64 *value,
    >> {
    >> unsigned long flags;
    >> int ret = 0;
    >> + bool is_any_cpu = !!(event->group_caps & PERF_EV_CAP_READ_ANY_CPU);
    >>
    >> /*
    >> * Disabling interrupts avoids all counter scheduling (context
    >> @@ -3600,7 +3605,8 @@ int perf_event_read_local(struct perf_event
    >> *event, u64 *value,
    >>
    >> /* If this is a per-CPU event, it must be for this CPU */
    >> if (!(event->attach_state & PERF_ATTACH_TASK) &&
    >> - event->cpu != smp_processor_id()) {
    >> + event->cpu != smp_processor_id() &&
    >> + !is_any_cpu) {
    >> ret = -EINVAL;
    >> goto out;
    >> }
    >> @@ -3610,7 +3616,7 @@ int perf_event_read_local(struct perf_event
    >> *event, u64 *value,
    >> * or local to this CPU. Furthermore it means its ACTIVE (otherwise
    >> * oncpu == -1).
    >> */
    >> - if (event->oncpu == smp_processor_id())
    >> + if (event->oncpu == smp_processor_id() || is_any_cpu)
    >> event->pmu->read(event);
    >>
    >> *value = local64_read(&event->count);
    >
    > And why are you modifying read_local for this? That didn't support
    > ACTIVE_PKG, so why should it support this?

    Maybe I'll make a separate patch to first have perf_event_read_local()
    also handle ACTIVE_PACKAGE? Because in those cases, the smp call made by
    read_local is unnecessary too.

    >
    > And again, where are the users?

    The DynamIQ PMU driver would be the user.

    -Saravana

    \
     
     \ /
      Last update: 2018-02-27 02:54    [W:3.455 / U:0.004 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site