linux-kernel - Re: [PATCH] perf, x86: Fall back to vmalloc for BTS buffer allocation

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Date:	Wed, 10 Sep 2014 09:57:22 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, eranian@...gle.com,
	tglx@...utronix.de, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] perf, x86: Fall back to vmalloc for BTS buffer allocation

On Tue, Sep 09, 2014 at 05:03:21PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> On my workstation the order 4 BTS buffer allocation fails regularly
> after the system has been up for some time due to memory
> fragmentation.
> 
> BTS is virtual memory, so we can just fall back to vmalloc
> instead of failing.
> 
> Do this here.
> 
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/perf_event_intel_ds.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event_intel_ds.c b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> index 9dc4199..3cf5b74 100644
> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> @@ -312,8 +312,11 @@ static int alloc_bts_buffer(int cpu)
>  
>  	buffer = kzalloc_node(BTS_BUFFER_SIZE, GFP_KERNEL | __GFP_NOWARN, node);
>  	if (unlikely(!buffer)) {
> -		WARN_ONCE(1, "%s: BTS buffer allocation failure\n", __func__);
> -		return -ENOMEM;
> +		buffer = vmalloc_node(BTS_BUFFER_SIZE, node);
> +		if (!buffer) {
> +			WARN_ONCE(1, "%s: BTS buffer allocation failure\n", __func__);
> +			return -ENOMEM;
> +		}
>  	}
>  
>  	max = BTS_BUFFER_SIZE / BTS_RECORD_SIZE;

We did this once, and that blew up big time.

Even now, vmalloc_fault() has a very explicit:

  WARN_ON_ONCE(in_nmi());

So this isn't going to happen until you fix that.

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