Re: [PATCH v2] io_uring: add IORING_ENTER_NO_IOWAIT to not set in_iowait

[Jens Axboe <axboe@kernel.dk>]

On 8/16/24 4:36 PM, David Wei wrote:
> io_uring sets current->in_iowait when waiting for completions, which
> achieves two things:
> 
> 1. Proper accounting of the time as iowait time
> 2. Enable cpufreq optimisations, setting SCHED_CPUFREQ_IOWAIT on the rq
> 
> For block IO this makes sense as high iowait can be indicative of
> issues. But for network IO especially recv, the recv side does not
> control when the completions happen.
> 
> Some user tooling attributes iowait time as CPU utilisation i.e. not
> idle, so high iowait time looks like high CPU util even though the task
> is not scheduled and the CPU is free to run other tasks. When doing
> network IO with e.g. the batch completion feature, the CPU may appear to
> have high utilisation.
> 
> This patchset adds a IOURING_ENTER_NO_IOWAIT flag that can be set on
> enter. If set, then current->in_iowait is not set. By default this flag
> is not set to maintain existing behaviour i.e. in_iowait is always set.
> This is to prevent waiting for completions being accounted as CPU
> utilisation.
> 
> Not setting in_iowait does mean that we also lose cpufreq optimisations
> above because in_iowait semantics couples 1 and 2 together. Eventually
> we will untangle the two so the optimisations can be enabled
> independently of the accounting.
> 
> IORING_FEAT_IOWAIT_TOGGLE is returned in io_uring_create() to indicate
> support. This will be used by liburing to check for this feature.

LGTM

-- 
Jens Axboe