Date: Mon, 15 Jan 2001 13:51:52 -0800 (PST) From: John Baldwin <jhb@FreeBSD.org> To: "Justin T. Gibbs" <gibbs@scsiguy.com> Cc: Peter Wemm <peter@netplex.com.au>, Poul-Henning Kamp <phk@critter.freebsd.dk>, Wilko Bulte <wkb@freebie.demon.nl>, cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org, Bruce Evans <bde@zeta.org.au> Subject: Re: cvs commit: src/sys/i386/conf GENERIC Message-ID: <XFMail.010115135152.jhb@FreeBSD.org> In-Reply-To: <200101152138.f0FLcLs61722@aslan.scsiguy.com>
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On 15-Jan-01 Justin T. Gibbs wrote:
>>> All that is required in the userland implementation is the setting of a
>>> flag so the userland thread scheduler does not perform a thread switch.
>>> Having an interrupt fire does not have the same consequences on a userland
>>> program as it does for the kernel.
>>
>>Actually, the process needs to not be switched. This is part of KSE, so you
>>would have to set a kernel flag in the kse for this, but yes, that would
>>work.
>>
>>Granted, it pessimizes the non-i386 case, but not that badly. The kernel
>>trap
>>to emulate only pessimizes the i386 case (though the 386 could do without
>>extr
>>a pessimizations, and it is a bigger pessimization.)
>
> I suppose I haven't read enough of the KSE stuff to understand why
> you need to not change processes (or is a process the "unit" used
> to represent a KSE?). Doesn't the userland scheduler decide what
> to do with any KSE it is given?
KSE doesn't have a userland scheduler. It's all in the kernel. But yes, you
are right, it is just the KSE, and not the process switch that needs to be
prevented.
> Anyway, if you use the "patch up the atomic operation" approach, you
> don't pessimize anyone except on the first run through a particular
> atomic operation.
Actually, the easiest way I can think of to do this would be to have an
internal cmpset() function:
int
cmpset(dst, exp, src)
{
if (on_a_386) {
set_no_switch_flag;
if (*dst == exp) {
*dst = src;
return 1;
} else
return 0;
} else
return atomic_cmpset(dst, exp, src);
}
Then change ast() in the kernel to check the per-KSE flag.
> --
> Justin
--
John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
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