[PATCH] nvme-pci: trigger disk activity LED

[Ming Lei]

On Tue, Mar 1, 2022 at 11:30 AM Enzo Matsumiya <ematsumiya at suse.de> wrote:
>
> On 02/28, Jens Axboe wrote:
> >On 2/28/22 2:22 AM, Christoph Hellwig wrote:
> >> I don't think we should add code to the absolutel fast path for
> >> blinkenlights.
> >
> >Agree. It'd be a lot better to put the cost on the led trigger
> >side, and not need anything in the fast path for block devices.
> >Monitor disk stats, or something like that.
>
> There's been at least 4 attempts to do so, as far as I'm aware (one of
> them being mine). All got rejected due to the complexity it introduced,
> that's how I ended up with this one-liner.
>
> Performance-wise, I'm understand the problems, but according to ftrace,
> ledtrig_disk_activity() adds an average of 0.2us overhead, whether an
> LED is assigned or not. Is that really unacceptable?
>
> If so, would introducing a CONFIG_NVME_LED (default =n) and wrap that
> call around it make it better? Then at least there's a chance to inform
> users that desires this feature about performance costs.

Another solution is to do it in userspace by tracking iostat in a fixed period,
such as, triggering one led activity if any read/write IO happens during 0.5sec.

Thanks,