Re: [RFC 0/2] optimise local-tw task resheduling

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On 3/13/23 14:16, Jens Axboe wrote:
On 3/12/23 9:45?PM, Pavel Begunkov wrote:
Didn't take a closer look just yet, but I grok the concept. One
immediate thing I'd want to change is the FACILE part of it. Let's call
it something a bit more straightforward, perhaps LIGHT? Or LIGHTWEIGHT?

I don't really care, will change, but let me also ask why?
They're more or less synonyms, though facile is much less
popular. Is that your reasoning?

Yep, it's not very common and the name should be self-explanatory
immediately for most people.

That's exactly the problem. Someone will think that it's
like normal tw but "better" and blindly apply it. Same happened
before with priority tw lists.

But the way to fix that is not through obscure naming, it's through
better and more frequent review. Naming is hard, but naming should be
basically self-explanatory in terms of why it differs from not setting
that flag. LIGHTWEIGHT and friends isn't great either, maybe it should
just be explicit in that this task_work just posts a CQE and hence it's
pointless to wake the task to run it unless it'll then meet the criteria
of having that task exit its wait loop as it now has enough CQEs
available. IO_UF_TWQ_CQE_POST or something like that. Then if it at some

There are 2 expectations (will add a comment)
1) it's posts no more that 1 CQE, 0 is fine

2) it's not urgent, including that it doesn't lock out scarce
[system wide] resources. DMA mappings come to mind as an example.

IIRC is a problem even now with nvme passthrough and DEFER_TASKRUN

point gets modified to also encompass different types of task_work that
should not cause wakes, then it can change again. Just tossing
suggestions out there...

I honestly don't see how LIGHTWEIGHT is better. I think a proper
name would be _LAZY_WAKE or maybe _DEFERRED_WAKE. It doesn't tell
much about why you would want it, but at least sets expectations
what it does. Only needs a comment that multishot is not supported.

--
Pavel Begunkov



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