Re: Interrupts vs signals

On 02/12/2024 12:42, Heikki Linnakangas wrote:
> On 02/12/2024 09:32, Thomas Munro wrote:
>> On Sat, Nov 23, 2024 at 10:58 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
>> wrote:
>>> Hmm, so this would replace the maybeSleepingOnInterrupts bitmask I
>>> envisioned. Makes a lot of sense. If it's a single bit though, that
>>> means that you'll still get woken up by interrupts that you're not
>>> waiting for. Maybe that's fine. Or we could merge the
>>> maybeSleepingOnInterrupts and pendingInterrupts bitmasks to a single
>>> atomic word, so that you would have a separate "maybe sleeping" bit for
>>> each interrupt bit, but could still use atomic_fetch_or atomically read
>>> the interrupt bits and announce the sleeping.
>>
>> I think one bit is fine for now.  At least, until we have a serious
>> problem with interrupts arriving when you're sleeping but not ready to
>> service that particular interrupt.  The 'interrupt bit already set,
>> don't try to wake me' stuff discussed earlier would limit the number
>> of useless wakeups to one, until you eventually are ready and consume
>> the interrupt.  The main case I can think of, if we fast forward to
>> the all-procsignals-become-interrupts patch (which I'll be rebasing on
>> top of this when the next version appears), is that you might receive
>> a sinval catchup request, but you might be busy running a long query.
>> Sinval catchup messages are only processed between queries, so you
>> just keep ignoring them until end of query.  I think that's fine, and
>> unlikely.  Do you have other cases in mind?
>
> Yeah, no, I think one bit is is good enough. Let's go with that.

Here's a new patch set version, with the following changes:

- Implement the "maybe sleeping" flag as a single bit in the pending
interrupts mask, per above discussion. One notable change is that I
moved the check for whether an interrupt is set out of the loop in
WaitEventSetWait(). It seemed redundant; all the WaitEventSetWaitBlock()
implementations also check the interrupt mask if the wakeup is received.
I'm sure it doesn't make a difference from performance point of view,
but it feels more natural to me this way.

- Suppress the wakeup in SendInterrupt if the interrupt was already
pending, per discussion

- Rename INTERRUPT_GENERAL_WAKEUP to INTERRUPT_GENERAL per Robert's
suggestion

- Fix a bunch of minor comment issues, some pointed out off-list by
Álvaro (thanks!)

--
Heikki Linnakangas
Neon (https://neon.tech)