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SubjectRe: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes to use function calls [POST 2]
Ingo Molnar wrote:
> nice stuff!
>
> I suspect the extra cost might be worth it for two reasons: 1) we could
> optimize the cross-call implementation further

Unfortunately, I think the kmalloc fix for the RCU issue is going to
hurt quite a lot.

> 2) on systems where TLB
> flushes actually matter, the ability to overlap multiple TLB flushes to
> the same single CPU might improve workloads.
>

...perhaps.

> FYI, i've created a new -tip topic for your patches, tip/x86/tlbflush.
> It's based on tip/irq/sparseirq (there are a good deal of dependencies
> with that topic).
>

Really? I didn't see much conflict when rebasing onto current tip.git.
Just an incidental context conflict in entry_arch.h.

> It would be nice to see some numbers on sufficiently SMP systems, using
> some mmap/munmap intense workload.

I've attached my test program: tlb-mash.c. Compile with "gcc -o
tlb-mash tlb-mash.c -lpthread" and run with ./tlb-mash X, where X is the
number of threads to run (2x cpus works well). It keeps running until
killed, with each thread repeatedly mprotecting a page within a shared
mapping.

J
#include <stdio.h>
#include <pthread.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/mman.h>

#define MAX_THREADS 256

static char *mapping;

static void *masher(void *v)
{
int id = (int)v;
unsigned offset = id * getpagesize();

printf("started thread %d\n", id);

for(;;) {
mprotect(mapping+offset, getpagesize(), PROT_READ);
mprotect(mapping+offset, getpagesize(), PROT_READ | PROT_WRITE);
}

return NULL;
}

int main(int argc, char **argv)
{
int i;
int nthreads = 4;
pthread_t threads[MAX_THREADS];

if (argc == 2) {
int t = atoi(argv[1]);
if (t != 0)
nthreads = t;
}
if (nthreads > MAX_THREADS)
nthreads = MAX_THREADS;

printf("creating %d threads...\n", nthreads);

mapping = mmap(0, getpagesize() * nthreads, PROT_NONE,
MAP_POPULATE|MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
if (mapping == (char *)-1) {
perror("mmap failed");
return 1;
}

for(i = 0; i < nthreads; i++) {
int ret;
ret = pthread_create(&threads[i], NULL, masher, (void *)i);
if (ret) {
printf("pthread create %d failed: %s\n", i, strerror(ret));
return 1;
}
}

for(i = 0; i < nthreads; i++) {
void *ret;
pthread_join(threads[i], &ret);
}

return 0;
}
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