Re: [PATCH] nvme-pci: trigger disk activity LED

[Enzo Matsumiya]

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.


Cheers,

Enzo