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Date:   Tue, 4 Apr 2023 20:45:29 +0200
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Subject: Re: [PATCH v5 02/19] block: introduce ioprio hints
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To:     Niklas Cassel <nks@flawful.org>, Jens Axboe <axboe@kernel.dk>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        linux-kernel@vger.kernel.org
Cc:     Bart Van Assche <bvanassche@acm.org>,
        Christoph Hellwig <hch@lst.de>,
        Damien Le Moal <damien.lemoal@opensource.wdc.com>,
        linux-scsi@vger.kernel.org, linux-ide@vger.kernel.org,
        linux-block@vger.kernel.org, Niklas Cassel <niklas.cassel@wdc.com>
References: <20230404182428.715140-1-nks@flawful.org>
 <20230404182428.715140-3-nks@flawful.org>
From:   Hannes Reinecke <hare@suse.de>
In-Reply-To: <20230404182428.715140-3-nks@flawful.org>
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On 4/4/23 20:24, Niklas Cassel wrote:
> From: Damien Le Moal <damien.lemoal@opensource.wdc.com>
> 
> IO priorities currently only use 6-bits of the 16-bits ioprio value: the
> 3-upper bits are used to define up to 8 priority classes (4 of which are
> valid) and the 3 lower bits of the value are used to define a priority
> level for the real-time and best-effort class.
> 
> The remaining 10-bits between the IO priority class and level are
> unused, and in fact, cannot be used by the user as doing so would
> either result in the value being completely ignored, or in an error
> returned by ioprio_check_cap().
> 
> Use these 10-bits of an ioprio value to allow a user to specify IO
> hints. An IO hint is defined as a 10-bits value, allowing up to 1023
> different hints to be specified, with the value 0 being reserved as the
> "no hint" case. An IO hint can apply to any IO that specifies a valid
> priority class other than NONE, regardless of the IO priority level
> specified.
> 
> To do so, the macros IOPRIO_PRIO_HINT() and IOPRIO_PRIO_VALUE_HINT() are
> introduced in include/uapi/linux/ioprio.h to respectively allow a user
> to get and set a hint in an ioprio value.
> 
> To support the ATA and SCSI command duration limits feature, 7 hints
> are defined: IOPRIO_HINT_DEV_DURATION_LIMIT_1 to
> IOPRIO_HINT_DEV_DURATION_LIMIT_7, allowing a user to specify which
> command duration limit descriptor should be applied to the commands
> serving an IO. Specifying these hints has for now no effect whatsoever
> if the target block devices do not support the command duration limits
> feature. However, in the future, block IO schedulers can be modified to
> optimize IO issuing order based on these hints, even for devices that
> do not support the command duration limits feature.
> 
> Given that the 7 duration limits hints defined have no effect on any
> block layer component, the actual definition of the duration limits
> implied by these hints remains at the device level.
> 
> Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
> Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
> ---
>   include/uapi/linux/ioprio.h | 49 +++++++++++++++++++++++++++++++++++++
>   1 file changed, 49 insertions(+)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes