Re: [patch] x86, perf: avoid spamming kernel log for bts buffer failure

[Stephane Eranian]

On Tue, Jul 1, 2014 at 11:34 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Jun 30, 2014 at 04:04:08PM -0700, David Rientjes wrote:
>> It's unnecessary to excessively spam the kernel log anytime the BTS buffer
>> cannot be allocated, so make this allocation __GFP_NOWARN.
>>
>> The user probably will want to at least find some artifact that the
>> allocation has failed in the past, probably due to fragmentation because
>> of its large size, when it's not allocated at bootstrap.  Thus, add a
>> WARN_ONCE() so something is left behind for them to understand why perf
>> commnads that require PEBS is not working properly.
>
> Can you elaborate a bit under which conditions this triggered? Typically
> we should be doing fairly well allocating such buffers with GFP_KERNEL,
> that should allow things like compaction to run and create higher order
> pages.
>
I think this triggers when you have fragmented memory and you have
perf_events active and inactive (i.e., 0 users = no nmi watchdog) frequently.
Each first user invokes the reserve_ds() function to reserve DS, PEBS, BTS.

The reason for BTS rather then PEBS is the size of the allocation.
PEBS allocates one page, i.e., less likely to get a failure than BTS
which allocates 4 pages, I think.

David and I discussed this. He can probably add more background
info, if needed.

> And the BTS (branch trace store) isn't _that_ large.
>
> That said, the patch is reasonable; although arguably we should maybe do
> the same to alloc_pebs_buffer().
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