linux-kernel - Re: [PATCH 07/15] mm: Add ___GFP

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Date:	Mon, 2 Mar 2015 11:37:01 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:	masami.hiramatsu.pt@...achi.com, namhyung@...nel.org,
	andi@...stfloor.org, ast@...mgrid.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 07/15] mm: Add ___GFP_NOTRACE

On Mon,  2 Mar 2015 10:01:00 -0600
Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:

> Add a gfp flag that allows kmalloc() et al to be used in tracing
> functions.
> 
> The problem with using kmalloc for tracing is that the tracing
> subsystem should be able to trace kmalloc itself, which it can't do
> directly because of paths like kmalloc()->trace_kmalloc()->kmalloc()
> or kmalloc()->trace_mm_page_alloc()->kmalloc().

This part I don't like at all. Why can't the memory be preallocated
when the hist is created (the echo 'hist:...')?

kmalloc must never be called from any tracepoint callback.

This change is currently a showstopper.

-- Steve


> 
> With this flag, tracing code could use a special version of kmalloc()
> that sets __GFP_NOTRACE on every allocation it does, while leaving the
> normal kmalloc() path untouched.
> 
> This would allow any tracepoint in the kmalloc path to be avoided via
> DEFINE_EVENT_CONDITION() redefinitions of those events, which check
> for ___GFP_NOTRACE immediately in their execution and break if set,
> thereby avoiding the recursion.
> 
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