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

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Date:   Tue, 1 Mar 2022 00:30:01 -0300
From:   Enzo Matsumiya <ematsumiya@...e.de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Christoph Hellwig <hch@....de>, linux-nvme@...ts.infradead.org,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Sagi Grimberg <sagi@...mberg.me>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nvme-pci: trigger disk activity LED

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