linux-kernel - Re: [PATCH RFC] hist lookups

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Message-Id: <20181107.220424.2221817007547297022.davem@davemloft.net>
Date:   Wed, 07 Nov 2018 22:04:24 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     acme@...nel.org
Cc:     jolsa@...hat.com, linux-kernel@...r.kernel.org,
        namhyung@...nel.org, jolsa@...nel.org
Subject: Re: [PATCH RFC] hist lookups

From: Arnaldo Carvalho de Melo <acme@...nel.org>
Date: Wed, 7 Nov 2018 17:28:15 -0300

> So perhaps we should tell the kernel that is ok to lose SAMPLEs but not
> the other events, and make userspace ask for PERF_RECORD_!SAMPLE in all
> ring buffers? Duplication wouldn't be that much of a problem?

I think we should strive to keep the policy in userspace.

The kernel simply provides the events that happen, and the user's
job is to take the events in sort of a "high priority interrupt"
context and push the slow path processing to a separate thread of
execution where drop policy can be implemented.

Jiri's work provides a framework for exactly that.

So what we can have is:

	cpu1  cpu2  cpu3  cpu4  cpu5  cpu6 ... cpuN
	|     |     |     |     |     |        |
	----------------------------------------
	                  |
			  |
			  | single event ring buffer
			  |
			  |
		ultra-fast perf event dequeue
	     queues in-order to event processing
		          |
		  event processing slow path
	        prioritization and drop policy
		     histogram insert
		        etc.