On Wed, Jul 2, 2014 at 3:31 PM, Peter Zijlstra <pet...@infradead.org> wrote: > On Wed, Jul 02, 2014 at 03:16:40PM +0200, Stephane Eranian wrote: >> On Tue, Jul 1, 2014 at 11:34 AM, Peter Zijlstra <pet...@infradead.org> 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. > > Right, that'd suck. I suppose we could also change that to allocate the > DS resources on first demand and never free them again. > Some may argue that if you never use perf_event again, you are wasting (1 + 1 + 4) pages per CPU. That may not be okay on some systems.
But yes, it would avoid this problem and also take the penalty for the allocs only once. > So only allocate the PEBS buffer when we create the first PEBS event, > and idem for the BTS muck. > >> 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. > > Sure.. > >> David and I discussed this. He can probably add more background >> info, if needed. > > It would still be good to see why compaction etc is failing. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
