On Tue, Jul 1, 2014 at 11:34 AM, Peter Zijlstra <[email protected]> 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(). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
