On 2024-02-23 06:31, Jens Axboe wrote:
> On 2/22/24 10:40 PM, David Wei wrote:
>> diff --git a/include/linux/io_uring_types.h b/include/linux/io_uring_types.h
>> index bd7071aeec5d..57318fc01379 100644
>> --- a/include/linux/io_uring_types.h
>> +++ b/include/linux/io_uring_types.h
>> @@ -425,6 +425,9 @@ struct io_ring_ctx {
>> DECLARE_HASHTABLE(napi_ht, 4);
>> #endif
>>
>> + /* iowait accounting */
>> + bool iowait_enabled;
>> +
>
> Since this is just a single bit, you should put it in the top section
> where we have other single bits for hot / read-mostly data. This avoids
> needing something many cache lines away for the hotter wait path, and it
> avoids growing the struct as there's still plenty of space there for
> this.
Got it, moved it to the cacheline aligned hot/read-only struct.
$current_year is when I learnt about C bitfields.
>
>> diff --git a/io_uring/register.c b/io_uring/register.c
>> index 99c37775f974..7cbc08544c4c 100644
>> --- a/io_uring/register.c
>> +++ b/io_uring/register.c
>> @@ -387,6 +387,12 @@ static __cold int io_register_iowq_max_workers(struct io_ring_ctx *ctx,
>> return ret;
>> }
>>
>> +static int io_register_iowait(struct io_ring_ctx *ctx)
>> +{
>> + ctx->iowait_enabled = true;
>> + return 0;
>> +}
>> +
>> static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
>> void __user *arg, unsigned nr_args)
>> __releases(ctx->uring_lock)
>> @@ -563,6 +569,12 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
>> break;
>> ret = io_unregister_napi(ctx, arg);
>> break;
>> + case IORING_REGISTER_IOWAIT:
>> + ret = -EINVAL;
>> + if (arg || nr_args)
>> + break;
>> + ret = io_register_iowait(ctx);
>> + break;
>
>
> This only allows you to set it, not clear it. I think we want to make it
> pass in the value, and pass back the previous. Something ala:
>
> static int io_register_iowait(struct io_ring_ctx *ctx, int val)
> {
> int was_enabled = ctx->iowait_enabled;
>
> if (val)
> ctx->iowait_enabled = 1;
> else
> ctx->iowait_enabled = 0;
> return was_enabled;
> }
>
> and then:
>
> case IORING_REGISTER_IOWAIT:
> ret = -EINVAL;
> if (arg)
> break;
> ret = io_register_iowait(ctx, nr_args);
> break;
>
When I first thought about this I wondered how to pass a value like int
val through arg which is a void*. That's a clever use of nr_args.
> I'd also add a:
>
> Fixes: 8a796565cec3 ("io_uring: Use io_schedule* in cqring wait")
>
> and mark it for stable, so we at least attempt to make it something that
> can be depended on.
Sorry, what does "mark it for stable" mean?
>
