linux-kernel - Re: [RFC/PATCH] ftrace: Allow to change size of function graph filters

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Message-ID: <20170113235001.6f92fb28@vmware.local.home>
Date:   Fri, 13 Jan 2017 23:50:01 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC/PATCH] ftrace: Allow to change size of function graph
 filters

On Sat, 14 Jan 2017 13:20:00 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> But I'm not sure how to synchronize hash table manipulations.  It
> seems synchronize_sched() is not good enough for function graph
> tracer, right?  So I limited changing filter size only when no tracer
> is used, but is it ok to have the limitation when adding or removing
> an entry to/from the table?  If not, what can I do?

There's already a hash that function tracing uses. Look at the code in
ftrace_hash_move(). If you make it where the hash is always looked at
with preemption disabled, you can use synchronize_sched() to modify the
hash.

The trampolines are a bit trickier, because there's no way to disable
preemption or do any other synchronization when they are called.

-- Steve