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Date:   Fri, 14 Feb 2020 11:54:50 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
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Subject: Re: [RFC PATCH v1] pin_on_cpu: Introduce thread CPU pinning system
 call
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----- On Jan 30, 2020, at 6:10 AM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>> It brings an interesting idea to the table though. Let's assume for now that
>> the only intended use of pin_on_cpu(2) would be to allow rseq(2) critical
>> sections to update per-cpu data on specific cpu number targets. In fact,
>> considering that userspace can be preempted at any point, we still need a
>> mechanism to guarantee atomicity with respect to other threads running on
>> the same runqueue, which rseq(2) provides. Therefore, that assumption does
>> not appear too far-fetched.
>>
>> There are 2 scenarios we need to consider here:
>>
>> A) pin_on_cpu(2) targets a CPU which is not part of the affinity mask.
>>
>> This case is easy: pin_on_cpu can return an error, and the caller needs to act
>> accordingly (e.g. figure out that this is a design error and report it, or
>> decide that it really did not want to touch that per-cpu data that badly and
>> make the entire process fall-back to a mechanism which does not use per-cpu
>> data at all from that point onwards)
> 
> Affinity masks currently are not like process memory: there is an
> expectation that they can be altered from outside the process.

Yes, that's my main issue.

> Given that the caller may not have any ways to recover from the
> suggested pin_on_cpu behavior, that seems problematic.

Indeed.

> 
> What I would expect is that if pin_on_cpu cannot achieve implied
> exclusion by running on the associated CPU, it acquires a lock that
> prevents others pin_on_cpu calls from entering the critical section, and
> tasks in the same task group from running on that CPU (if the CPU
> becomes available to the task group).  The second part should maintain
> exclusion of rseq sequences even if their fast path is not changed.

I try to avoid mutual exclusion over shared memory as rseq fallback whenever
I can, so we can use rseq from lock-free algorithms without losing lock-freedom.

> (On the other hand, I'm worried that per-CPU data structures are a dead
> end for user space unless we get containerized affinity masks, so that
> contains only see resources that are actually available to them.)

I'm currently implementing a prototype of the following ideas, and I'm curious to
read your thoughts on those:

I'm adding a "affinity_pinned" flag to the task struct of each thread. It can
be set and cleared only by the owner thread through pin_on_cpu syscall commands.
When the affinity is pinned by a thread, trying to change its affinity (from an
external thread, or possibly from itself) will fail.

Whenever a thread would (temporarily) pin itself on a specific CPU, it would
also pin its affinity mask as a side-effect. When a thread unpins from a CPU,
the affinity mask stays pinned. The purpose of keeping this affinity pinned
state per-thread is to ensure we don't end up with tiny race windows where
changing the thread's affinity mask "typically" works, but fails once in a
while because it's done concurrently with a 1ms long cpu pinning. This would
lead to flaky code, and I try hard to avoid that.

How changing this affinity should fail (from sched_setaffinity and cpusets) is a
big unanswered question. I see two major alternatives so far:

1) We deliver a signal to the target thread (SIGKILL ? SIGSEGV ?), considering
   that failure to be able to change its affinity mask means we need to send a
   signal. How exactly would the killed application recover (or if it should)
   is still unclear.

2) Return an error to the sched_setaffinity or cpusets caller, and let it deal
   with the error as it sees fit: ignore it, log it, or send a signal.

I think option (2) provides the most flexiblity, and moves policy outside of
the kernel, which is a good thing. However, looking at how cpusets seems to
simply ignore errors when setting a task's cpumask, I wonder if asking from
cpusets to handle any kind of error is asking too much. :-/

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com