Re: [PATCH] block: enable passthrough command statistics

[Christoph Hellwig <hch@lst.de>]

On Wed, Oct 02, 2024 at 02:07:44PM -0700, Keith Busch wrote:
> +		accounting of the disk. Set to 0 to disable all stats. Set to 1
> +		to enable block IO stats. Set to 2 to enable passthrough stats
> +		in addition to block IO.

Jens' reply suggest he likes this interface, but I have to say I
already hated it with a passion for the merges - overloading a
previously boolean file with a numberic value is not exactly an
intuitive interface.  Is a new sysfs file for this really a problem?

> +	if (!bio)
> +		return false;
> +	if (!bio->bi_bdev)
> +		return false;
> +	if (blk_rq_bytes(req) & (bdev_logical_block_size(bio->bi_bdev) - 1))
> +		return false;

I understand why all these conditions are there, because basically
they'd break the current code to collect stats.  But I think this needs
a comment explaining why they are there, and why the statistics are
still useful without the requests matching them.

> +	lim = queue_limits_start_update(disk->queue);
> +	if (!ios)
> +		lim.features &= ~(BLK_FEAT_IO_STAT | BLK_FEAT_PASSTHROUGH_STAT);
> +	else if (ios == 2)
> +		lim.features |= BLK_FEAT_IO_STAT | BLK_FEAT_PASSTHROUGH_STAT;
> +	else if (ios == 1) {
> +		lim.features |= BLK_FEAT_IO_STAT;
> +		lim.features &= ~BLK_FEAT_PASSTHROUGH_STAT;

BLK_FEAT_IO_STAT is in ->features because drivers need to opt into it,
but BLK_FEAT_PASSTHROUGH_STAT is purely a flag triggered by sysfs and
should go into ->flags.