RE: [CFT][RFC] HT scheduler

[Nakajima, Jun]

> > Regarding the overhead of the shared runqueue lock:
> >
> > Is the "lock" prefix actually required for locking between x86
> > siblings which share the same L1 cache?
> 
> I bet it is. In a big way.

Of course it is.

> 
> The lock does two independent things:
>  - it tells the core that it can't just crack up the load and store.
>  - it also tells other memory ops that they can't re-order around it.
> 
> Neither of these have anything to do with the L1 cache.
> 
> In short, I'd be very very surprised if you didn't need a "lock"
prefix
> even between hyperthreaded cores. It might be true in some specific
> implementation of HT, but quite frankly I'd doubt it, and I'd be
willing
> to guarantee that Intel would never make that architectural even if it
was
> true today (ie it would then break on future versions).

Correct. If such a code happens to be working today, it would be broken
anytime.


> 
> It should be easy enough to test in user space.
> 
> [ Time passes ]
> 
> Done. Check this program out with and without the "lock ;" prefix.
With
> the "lock" it will run forever on a HT CPU. Without the lock, it will
show
> errors pretty much immediately when the two threads start accessing
"nr"
> concurrently.
> 
> 		Linus
> 
> ----
> #include <pthread.h>
> #include <signal.h>
> #include <unistd.h>
> #include <stdio.h>
> 
> unsigned long nr;
> 
> #define LOCK "lock ;"
> 
> void * check_bit(int bit)
> {
> 	int set, reset;
> 	do {
> 		asm(LOCK "btsl %1,%2; sbbl %0,%0": "=r" (set): "r"
(bit), "m"
> (nr):"memory");
> 		asm(LOCK "btcl %1,%2; sbbl %0,%0": "=r" (reset): "r"
(bit),
> "m" (nr):"memory");
> 	} while (reset && !set);
> 	fprintf(stderr, "bit %d: %d %d (%08x)\n", bit, set, reset, nr);
> 	return NULL;
> }
> 
> static void * thread1(void* dummy)
> {
> 	return check_bit(0);
> }
> 
> static void * thread2(void *dummy)
> {
> 	return check_bit(1);
> }
> 
> int main(int argc, char ** argv)
> {
> 	pthread_t p;
> 
> 	pthread_create(&p, NULL, thread1, NULL);
> 	sleep(1);
> 	thread2(NULL);
> 	return 1;
> }
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