linux-kernel - Re: [PATCH] scheduler: introduce SCHED_RESET_ON

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Date:	Sun, 7 Jun 2009 12:11:36 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Lennart Poettering <mzxreary@...inter.de>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: Re: [PATCH] scheduler: introduce SCHED_RESET_ON_FORK scheduling
	policy flag, Second try


* Lennart Poettering <mzxreary@...inter.de> wrote:

> On Fri, 05.06.09 13:54, Ingo Molnar (mingo@...e.hu) wrote:
> 
> > 
> > 
> > * Lennart Poettering <mzxreary@...inter.de> wrote:
> > 
> > > Why expose this via sched_setscheduler() and not other syscalls 
> > > such as prctl() or sched_setparam()?
> > > 
> > > prctl() does not take a pid parameter. Due to that it would be 
> > > impossible to modify this flag for other processes than the 
> > > current one.
> > > 
> > > The struct passed to sched_setparam() can unfortunately not be 
> > > extended without breaking compatibility, since sched_setparam() 
> > > lacks a size parameter.
> > 
> > Well, it could be extended, if we wanted to. Right now 
> > sched_priority has a valid value of 0 to 100. We could introduce 
> > a new value '-1' to mean: 'extended struct sched_param'.
> > 
> > A new getparam syscall could then be introduced - only used by 
> > new user-space.
> 
> The man page of sched_getparam() is pretty explicit in that the 
> sched_priority field of the struct must lie between 
> sched_get_priority_min() and sched_get_priority_max(). If you'd 
> overload sched_priority like this you might end up breaking 
> applications that rely on this, for example RT watchdogs that go 
> through /proc and query the scheduling parameters of all threads.

Only the new sched_gtparam syscall would do this - and that syscall 
would have to be propagated to glibc and user-space with appropriate 
care of course. (probably resulting in a separate function not a 
change/breakage of existing functionality)

Probably not worth the pain though.

	Ingo
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