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

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Date:	Mon, 2 Mar 2015 10:12:32 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Tom Zanussi <tom.zanussi@...ux.intel.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 07/15] mm: Add ___GFP_NOTRACE

On Mon, Mar 2, 2015 at 10:03 AM, Tom Zanussi
<tom.zanussi@...ux.intel.com> wrote:
> On Mon, 2015-03-02 at 09:58 -0800, Alexei Starovoitov wrote:
>> On Mon, Mar 2, 2015 at 8:46 AM, Tom Zanussi <tom.zanussi@...ux.intel.com> wrote:
>> > On Mon, 2015-03-02 at 11:37 -0500, Steven Rostedt wrote:
>> >> 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:...')?
>> >>
>> >
>> > Yeah, I didn't like it either.  My original version did exactly what you
>> > suggest and preallocated an array of entries to 'allocate' from in order
>> > to avoid the problem.
>> >
>> > But I wanted to attempt to use the bpf_map directly, which already uses
>> > kmalloc internally.  My fallback in case this wouldn't fly, which it
>> > obviously won't, would be to add an option to have the bpf_map code
>> > preallocate a maximum number of entries or pass in a client-owned array
>> > for the purpose.  I'll do that if I don't hear any better ideas..
>>
>> Tom, I'm still reading through the patch set.
>> Quick comment for the above.
>> Currently there are two map types: array and hash.
>> array type is pre-allocating all memory at map creation time.
>> hash is allocating on demand.
>
> OK, so would it make sense to do the same for the hash type, or at least
> add an option that does that?

I'm not sure what would be the meaning of hash map that has all
elements pre-allocated...
As I'm reading your cover letter, I agree, we need to find a way
to call kmalloc_notrace-like from tracepoints.
Not sure that patch 8 style of duplicating the functions is clean.
Can we keep kmalloc/kfree as-is and do something like
if (in_tracepoint()) check inside ftrace_raw_kmalloc*  ?
so that kmalloc will be traced but calls to kmalloc from inside
tracepoints will be automatically suppressed ?
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