Re: [PATCH 2/2] mm/vmstat: Protect per cpu variables with preempt disable on RT

From: Vlastimil Babka
Date: Wed Aug 04 2021 - 09:42:31 EST

On 8/4/21 11:54 AM, Mel Gorman wrote:
> On Wed, Aug 04, 2021 at 01:54:47AM +0200, Thomas Gleixner wrote:
>> <tglx> so in vmstat.c there is this magic comment:
>> <tglx> * For use when we know that interrupts are disabled
>> <tglx> * or when we know that preemption is disabled and that
>> <tglx> * particular counter cannot be updated from interrupt context.
>> <tglx> how can I know which counters need what?
>> <mm_expert> I don't think there's a list, one would have to check on counter to counter basis :/
>> <tglx> and of course there is nothing which validates that, right?
>> <mm_expert> exactly
> While I'm not "mm_expert", I agree with his/her statements.

Phew, since you do, I can now disclose it was me.

> Each counter
> would need to be audited and two question are asked
> o If this counter is inaccurate, does anything break?
> o If this counter is inaccurate, does it both increment and decrement
> allowing the possibility it goes negative?
> The decision on that is looking at the counter and seeing if any
> functional decision is made based on its value. So two examples;
> NR_VMSCAN_IMMEDIATE is a node-based counter that only every
> increments and is reported to userspace. No kernel code makes
> any decision based on its value. Therefore it's likely safe to
> move to numa_stat_item instead.
> Action: move it
> WORKINGSET_ACTIVATE_FILE is a node-based counter that is used to
> determine if a mem cgroup is potentially congested by looking at
> the ratio of cgroup to node refault rates as well as deciding if
> LRU file pages should be deactivate. If that value drifts, the
> ratios are miscalculated and could lead to functional oddities
> and therefore must be accurate.
> Action: leave it alone
> I guess it could be further split into state that must be accurate from
> IRQ and non-IRQ contexts but that probably would be very fragile and
> offer limited value.
>> Brilliant stuff which prevents you to do any validation on this. Over
>> the years there have been several issues where callers had to be fixed
>> by analysing bug reports instead of having a proper instrumentation in
>> that code which would have told the developer that he got it wrong.
> I'm not sure it could be validated at build-time but I'm just back from
> holiday and may be lacking imagination.

The idea was not build-time, but runtime (hidden behind lockdep, VM_DEBUG or
whatnot), i.e.:

<sched_expert> what that code needs is switch(item) { case foo1: case foo2:
lockdep_assert_irqs_disabled(); break; case bar1: case bar2:
lockdep_assert_preempt_disabled(); lockdep_assert_no_in_irq(); break; } or
something along those lines

>> Of course on RT kernels the preempt_disable_rt() will serialize
>> everything correctly, but as we have learned over the years just
>> slapping _if_rt() or if_not_rt() variants of things around is most of
>> the time papering over the underlying problem of badly defined
>> protection scopes. Let's not proliferate that. As I said in the above
>> IRC conversation:
>> <tglx> I fundamentally hate this preempt_disable_rt() muck
> The issue is that even if this was properly audited and the inaccurate
> and accurate counters were in the proper enums using the correct APIs, it
> would still be necessary to protect the accurate counters from updates from
> IRQ context. Hence, as I write this, I don't think preempt_[dis|en]able_rt
> would go away and that is why I didn't continue with the series to break
> out "accurate" stats