linux-kernel - Re: [RFC PATCH 1/2] bdev: add support for CPU latency PM QoS tuning

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Message-ID: <e5e97bad-075a-4d78-af78-3bbc124c06b1@kernel.dk>
Date: Thu, 29 Aug 2024 05:37:47 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Tero Kristo <tero.kristo@...ux.intel.com>
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] bdev: add support for CPU latency PM QoS tuning

On 8/29/24 1:18 AM, Tero Kristo wrote:
> diff --git a/block/bio.c b/block/bio.c
> index e9e809a63c59..6c46d75345d7 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -282,6 +282,8 @@ void bio_init(struct bio *bio, struct block_device *bdev, struct bio_vec *table,
>  	bio->bi_max_vecs = max_vecs;
>  	bio->bi_io_vec = table;
>  	bio->bi_pool = NULL;
> +
> +	bdev_update_cpu_latency_pm_qos(bio->bi_bdev);
>  }
>  EXPORT_SYMBOL(bio_init);

This is entirely the wrong place to do this, presumably it should be
done at IO dispatch time, not when something initializes a bio.

And also feels like entirely the wrong way to go about this, adding
overhead to potentially each IO dispatch, of which there can be millions
per second.

-- 
Jens Axboe