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Subject: Re: [PATCH] Add tracepoints to track pagecache transition
From: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Atsushi Tsuji <a-tsuji@bk.jp.nec.com>, linux-kernel@vger.kernel.org,
       Jason Baron <jbaron@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       Mathieu Desnoyers <compudj@krystal.dyndns.org>,
       "Frank Ch. Eigler" <fche@redhat.com>,
       Kazuto Miyoshi <miyoshi@linux.bs1.fc.nec.co.jp>, rostedt@goodmis.org,
       linux-mm <linux-mm@kvack.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Nick Piggin <nickpiggin@yahoo.com.au>, Hugh Dickins <hugh@veritas.com>,
       Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
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>> If you want to see I/O activity, you need to add tracepoint into block
>> layer.
>
> We already have that, Arnaldo converted blktrace to use tracepoints and
> wrote an ftrace tracer for it.

Yup. I agree I selected wrong word.
My point is, if he want to know I/O delaying reason, the number of the
cache pages seems unrelated thing.