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Date:   Tue, 13 Feb 2018 19:23:51 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Raghavendra Rao Ananta <rananta@codeaurora.org>
Cc:     mingo@redhat.com, acme@kernel.org,
        alexander.shishkin@linux.intel.com, jolsa@redhat.com,
        namhyung@kernel.org, linux-kernel@vger.kernel.org,
        psodagud@codeaurora.org, tsoni@codeaurora.org
Subject: Re: [PATCH] perf: Add support for creating offline events
Message-ID: <20180213182351.GQ25201@hirez.programming.kicks-ass.net>
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On Fri, Feb 09, 2018 at 03:07:00PM -0800, Raghavendra Rao Ananta wrote:
> Perf framework doesn't allow creation of hardware events if
> the requested CPU is offline. However, creation of an event
> is achievable if the event is attached to the PMU as soon
> as the CPU is online again.
> 
> So, introducing a feature that could allow to create events
> even when the CPU is offline and return a success to the caller.
> If, during the time of event creation, the CPU is found offline,
> the event is moved to a new state (PERF_EVENT_STATE_DORMANT). As
> and when the CPU is know to be woken up (through hotplug notifiers),
> all the dormant events would be attached to the PMU (by
> perf_install_in_context()). If during the life time of the event,
> the CPU hasn't come online, the dormant event would just be freed.

This is horrible.. and you seem to have forgotten to explain why you
care about offline CPUs.