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Date:   Tue, 21 Jan 2020 20:11:08 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Chris Lameter <cl@linux.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 21, 2020, at 4:44 PM, Chris Lameter cl@linux.com wrote:

> These scenarios are all pretty complex and will be difficult to understand
> for the user of these APIs.
> 
> I think the easiest solution (and most comprehensible) is for the user
> space process that does per cpu operations to get some sort of signal. If
> its not able to handle that then terminate it. The code makes a basic
> assumption after all that the process is running on a specific cpu. If
> this is no longer the case then its better to abort if the process cannot
> handle moving to a different processor.

The point of pin_on_cpu() is to allow threads to access per-cpu data
structures belonging to a given CPU even if they cannot run on that
CPU (because it is offline).

I am not sure what scenario your signal delivery proposal aims to cover.

Just to try to put this into the context of a specific scenario to see
if I understand your point, is the following what you have in mind ?

1. Thread A issues pin_on_cpu(5),
2. Thread B issues sched_setaffinity removing cpu 5 from thread A's
   affinity mask,
3. Noticing that it would generate an invalid combination, rather than
   failing sched_setaffinity, it would send a SIGSEGV (or other) signal
   to thread A.

Or so you have something entirely different in mind ?

Thanks,

Mathieu

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