Re: [PATCH 07/15] mm: Add ___GFP_NOTRACE

[Alexei Starovoitov]

On Mon, Mar 2, 2015 at 10:25 AM, Tom Zanussi
<tom.zanussi@xxxxxxxxxxxxxxx> wrote:
> On Mon, 2015-03-02 at 10:12 -0800, Alexei Starovoitov wrote:
>> On Mon, Mar 2, 2015 at 10:03 AM, Tom Zanussi
>> <tom.zanussi@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> 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...
>
> The idea would be that instead of getting your individually kmalloc'ed
> elements on-demand from kmalloc while in the handler, you'd get them
> from a pool you've pre-allocated when you set up the table.  This could
> be from a list of individual entries you've already kmalloc'ed ahead of
> time, or from an array of n * sizeof(entry).

would work, but kinda ugly, since we will pre-allocate a lot
and may not be using it at all.

> This would also allow you to avoid GFP_ATOMIC for those.
>
>> 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.
>
> No, it's horrible, but it does the job without changing the normal path
> at all.
>
>> Can we keep kmalloc/kfree as-is and do something like
>> if (in_tracepoint()) check inside ftrace_raw_kmalloc*  ?
>
> Yeah, that's essentially what TP_CONDITION() in patch 8 (Make kmem
> memory allocation tracepoints conditional) does.

not quite, I mean something like global kmalloc recursion flag.
then kmalloc doesn't need to change.
ftrace_raw_event_kmem_alloc() would use a flag
inside ftrace_event_call struct or global recursion flag.
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