On Thu, Feb 15, 2018 at 03:01:41PM -0800, Raghavendra Rao Ananta wrote:
> Perf framework doesn't allow prevserving CPU events across
> CPU hotplugs. The events are scheduled out as and when the
> CPU walks offline. Moreover, the framework also doesn't
> allow the clients to create events on an offline CPU. As
> a result, the clients have to keep on monitoring the CPU
> state until it comes back online.
>
> Therefore, introducing the perf framework to support creation
> and preserving of (CPU) events for offline CPUs. Through
> this, the CPU's online state would be transparent to the
> client and it not have to worry about monitoring the CPU's
> state. Success would be returned to the client even while
> creating the event on an offline CPU. If during the lifetime
> of the event the CPU walks offline, the event would be
> preserved and would continue to count as soon as (and if) the
> CPU comes back online.
>
> Signed-off-by: Raghavendra Rao Ananta <rana...@codeaurora.org>
> ---
> include/linux/perf_event.h | 7 +++
> kernel/events/core.c | 123
> +++++++++++++++++++++++++++++++++------------
> 2 files changed, 97 insertions(+), 33 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index 7546822..bc07f16 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -489,6 +489,7 @@ struct perf_addr_filters_head {
> * enum perf_event_state - the states of a event
> */
> enum perf_event_state {
> + PERF_EVENT_STATE_DORMANT = -5,
> PERF_EVENT_STATE_DEAD = -4,
> PERF_EVENT_STATE_EXIT = -3,
> PERF_EVENT_STATE_ERROR = -2,
> @@ -687,6 +688,12 @@ struct perf_event {
> #endif
>
> struct list_head sb_list;
> +
> + /* Entry into the list that holds the events whose CPUs
> + * are offline. These events will be removed from the
> + * list and installed once the CPU wakes up.
> + */
> + struct list_head dormant_entry;
No this is absolutely disguisting. You can simply keep the events in the
dead CPU's context. It's really not that hard.
Also, you _still_ don't explain why you care about dead CPUs.
