Re: [PATCH] iommu/arm-smmu-v3: add tracepoints for cmdq_issue_cmdlist

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Fri, Aug 28, 2020 at 07:55:18AM +0000, Song Bao Hua (Barry Song) wrote:
> 
> 
> > -----Original Message-----
> > From: Jean-Philippe Brucker [mailto:jean-philippe@linaro.org]
> > Sent: Friday, August 28, 2020 7:41 PM
> > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
> > Cc: iommu@lists.linux-foundation.org; linux-arm-kernel@lists.infradead.org;
> > robin.murphy@arm.com; will@kernel.org; Linuxarm <linuxarm@huawei.com>
> > Subject: Re: [PATCH] iommu/arm-smmu-v3: add tracepoints for
> > cmdq_issue_cmdlist
> > 
> > Hi,
> > 
> > On Thu, Aug 27, 2020 at 09:33:51PM +1200, Barry Song wrote:
> > > cmdq_issue_cmdlist() is the hotspot that uses a lot of time. This
> > > patch adds tracepoints for it to help debug.
> > >
> > > Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> > > ---
> > >  * can furthermore develop an eBPF program to benchmark using this
> > > trace
> > 
> > Have you tried using kprobe and kretprobe instead of tracepoints?
> > Any noticeable performance drop?
> 
> Yes. Pls read this email.
> kprobe overhead and OPTPROBES implementation on ARM64
> https://www.spinics.net/lists/arm-kernel/msg828788.html

Thanks for the pointer. I'm asking because I've been working on BPF
trampolines (fentry/fexit) for arm64, which reduce the overhead of BPF
tracing, and am looking for confirmation that it's needed before spending
more time on it. Perhaps OPTPROBE would be a better first step, though, it
seems more generic.

Thanks,
Jean

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