On 7/23/20 3:58 PM, peterz@infradead.org wrote:

On Thu, Jul 23, 2020 at 03:04:13PM -0400, Waiman Long wrote:

quoted

On 7/23/20 2:47 PM, peterz@infradead.org wrote:

quoted

On Thu, Jul 23, 2020 at 02:32:36PM -0400, Waiman Long wrote:

quoted

BTW, do you have any comment on my v2 lock holder cpu info qspinlock patch?
I will have to update the patch to fix the reported 0-day test problem, but
I want to collect other feedback before sending out v3.

I want to say I hate it all, it adds instructions to a path we spend an
aweful lot of time optimizing without really getting anything back for
it.

It does add some extra instruction that may slow it down slightly, but I
don't agree that it gives nothing back. The cpu lock holder information can
be useful in analyzing crash dumps and in some debugging situation. I think
it can be useful in RHEL for this readon. How about an x86 config option to
allow distros to decide if they want to have it enabled? I will make sure
that it will have no performance degradation if the option is not enabled.

Config knobs suck too; they create a maintenance burden (we get to make
sure all the permutations works/build/etc..) and effectively nobody uses
them, since world+dog uses what distros pick.

Anyway, instead of adding a second per-cpu variable, can you see how
horrible something like this is:

unsigned char adds(unsigned char var, unsigned char val)
{
	unsigned short sat = 0xff, tmp = var;

	asm ("addb	%[val], %b[var];"
	     "cmovc	%[sat], %[var];"
	     : [var] "+r" (tmp)
	     : [val] "ir" (val), [sat] "r" (sat)
	     );

	return tmp;
}

Another thing to try is, instead of threading that lockval throughout
the thing, simply:

#define _Q_LOCKED_VAL	this_cpu_read_stable(cpu_sat)

or combined with the above

#define _Q_LOCKED_VAL	adds(this_cpu_read_stable(cpu_number), 2)

and see if the compiler really makes a mess of things.

Thanks for the suggestion. I will try that out.

Cheers,
Longman