RE: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer

[Metzger, Markus T]

>-----Original Message-----
>From: Peter Zijlstra [mailto:a.p.zijlstra@xxxxxxxxx]
>Sent: Monday, April 27, 2009 12:54 PM
>To: Ingo Molnar
>Cc: Andrew Morton; Metzger, Markus T; markus.t.metzger@xxxxxxxxx; roland@xxxxxxxxxx;
>eranian@xxxxxxxxxxxxxx; oleg@xxxxxxxxxx; Villacis, Juan; ak@xxxxxxxxxxxxxxxxxx; linux-
>kernel@xxxxxxxxxxxxxxx; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx
>Subject: Re: [rfc 2/2] x86, bts: use physically non-contiguous trace buffer
>
>On Fri, 2009-04-24 at 10:31 +0200, Ingo Molnar wrote:
>> * Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>
>> > The patch looks like a regression to me.  vmalloc memory is slower
>> > to allocate, slower to free, slower to access and can exhaust or
>> > fragment the vmalloc arena.  Confused.
>>
>> Performance does not matter here (this is really a slowpath), but
>> fragmentation does matter, especially on 32-bit systems.
>>
>> I'd not uglify the code via vmap() - and vmap has the same
>> fundamental address space limitations on 32-bit as vmalloc().
>>
>> The existing kmalloc() is fine. We do larger than PAGE_SIZE
>> allocations elsewhere too (the kernel stack for example), and this
>> is a debug facility, so failing the allocation is not a big problem
>> even if it happens.
>
>Nobody has yet told what the typical size of these allocations are. If
>they're large enough to account in pages, one should arguable use the
>page allocator not kmalloc. Also, any >3 order allocation (>32kb) are
>very likely to fail. Having this ptrace interface work in the (unloaded)
>development environment but not in a (loaded) production environment
>will render it basically useless IMHO.

The size of these allocations is limited by the lockable memory ulimit
and debuggers typically need to divide this between multiple threads.

In practice, I would expect the buffers to be small. One page already
gives you ~170 entries and the most interesting part is the tail of
the trace, anyway.


>Having a regular high order allocation with vmalloc/vmap fallback is
>quite normal, esp. if one wants to promote the use of this facility as
>usable.
>
>So, no, I very strongly disagree that the existing kmalloc is fine.

I could add the vmalloc fallback. Is there already some function that
tries kmalloc and falls back to vmalloc which I should use?

thanks,
markus.
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