Re: [PATCH next v1 2/2] io_uring: limit local tw done

[Pavel Begunkov <asml.silence@gmail.com>]

On 11/21/24 14:31, Jens Axboe wrote:
> On 11/21/24 7:25 AM, Pavel Begunkov wrote:
>> On 11/21/24 01:12, Jens Axboe wrote:
>>> On 11/20/24 4:56 PM, Pavel Begunkov wrote:
>>>> On 11/20/24 22:14, David Wei wrote:
...
>>> I think that can only work if we change work_llist to be a regular list
>>> with regular locking. Otherwise it's a bit of a mess with the list being
>>
>> Dylan once measured the overhead of locks vs atomics in this
>> path for some artificial case, we can pull the numbers up.
> 
> I did it more recently if you'll remember, actually posted a patch I
> think a few months ago changing it to that. But even that approach adds

Right, and it's be a separate topic from this set.

> extra overhead, if you want to add it to the same list as now you need

Extra overhead to the retry path, which is not the hot path,
and coldness of it is uncertain.

> to re-grab (and re-disable interrupts) the lock to add it back. My gut
> says that would be _worse_ than the current approach. And if you keep a
> separate list instead, well then you're back to identical overhead in
> terms of now needing to check both when needing to know if anything is
> pending, and checking both when running it.
> 
>>> reordered, and then you're spending extra cycles on potentially
>>> reordering all the entries again.
>>
>> That sucks, I agree, but then it's same question of how often
>> it happens.
> 
> At least for now, there's a real issue reported and we should fix it. I
> think the current patches are fine in that regard. That doesn't mean we
> can't potentially make it better, we should certainly investigate that.
> But I don't see the current patches as being suboptimal really, they are
> definitely good enough as-is for solving the issue.

That's fair enough, but I still would love to know how frequent
it is. There is no purpose in optimising it as hot/slow path if
it triggers every fifth run or such. David, how easy it is to
get some stats? We can hack up some bpftrace script

-- 
Pavel Begunkov