[PATCH] context-switching overhead in X, ioport()

while debugging/improving scheduling latencies i got the following
strange latency report from Lee Revell:

  http://krustophenia.net/testresults.php?dataset=2.6.8.1-P6#/var/www/2.6.8.1-P6

this trace shows a 120 usec latency caused by XFree86, on a 600 MHz x86
system. Looking closer reveals:

  00000002 0.006ms (+0.003ms): __switch_to (schedule)
  00000002 0.088ms (+0.082ms): finish_task_switch (schedule)

it took more than 80 usecs for XFree86 to do a context-switch!

it turns out that the reason for this (massive) context-switching
overhead is the following change in 2.6.8:

      [PATCH] larger IO bitmaps

To demonstrate the effect of this change i've written ioperm-latency.c
(attached), which gives the following on vanilla 2.6.8.1:

  # ./ioperm-latency
  default no ioperm:             scheduling latency: 2528 cycles
  turning on port 80 ioperm:     scheduling latency: 10563 cycles
  turning on port 65535 ioperm:  scheduling latency: 10517 cycles

the ChangeSet says:

        Now, with the lazy bitmap allocation and per-CPU TSS, this
        will really not drain any resources I think.

this is plain wrong. An increase in the IO bitmap size introduces
per-context-switch overhead as well: we now have to copy an 8K bitmap
every time XFree86 context-switches - even though XFree86 never uses
ports higher than 1024! I've straced XFree86 on a number of x86 systems
and in every instance ioperm() was used - so i'd say the majority of x86
Linux systems running 2.6.8.1 are affected by this problem.

This not only causes lots of overhead, it also trashes ~16K out of the
L1 and L2 caches, on every context-switch. It's as if XFree86 did a L1
cache flush on every context-switch ...

the simple solution would be to revert IO_BITMAP_BITS back to 1024 and
release 2.6.8.2?

I've implemented another solution as well, which tracks the
highest-enabled port # for every task and does the copying of the bitmap
intelligently. (patch attached) The patched kernel gives:

  # ./ioperm-latency
  default no ioperm:             scheduling latency: 2423 cycles
  turning on port 80 ioperm:     scheduling latency: 2503 cycles
  turning on port 65535 ioperm:  scheduling latency: 10607 cycles

this is much more acceptable - the full overhead only occurs in the very
unlikely event of a task using the high ioport range. X doesnt suffer
any significant overhead.

(tracking the maximum allowed port # also allows a simplification of
io_bitmap handling: e.g. we dont do the invalid-offset trick anymore -
the IO bitmap in the TSS is always valid and secure.)

I tested the patch on x86 SMP and UP, it works fine for me. I tested
boundary conditions as well, it all seems secure.

	Ingo

#include <errno.h>
#include <stdio.h>
#include <sched.h>
#include <signal.h>
#include <sys/io.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/unistd.h>

#define CYCLES(x) asm volatile ("rdtsc" :"=a" (x)::"edx")

#define __NR_sched_set_affinity 241
_syscall3 (int, sched_set_affinity, pid_t, pid, unsigned int, mask_len, unsigned long *, mask)

/*
 * Use a pair of RT processes bound to the same CPU to measure
 * context-switch overhead:
 */
static void measure(void)
{
	unsigned long i, min = ~0UL, pid, mask = 1, t1, t2;

	sched_set_affinity(0, sizeof(mask), &mask);

	pid = fork();
	if (!pid)
		for (;;) {
			asm volatile ("sti; nop; cli");
			sched_yield();
		}

	sched_yield();
	for (i = 0; i < 100; i++) {
		asm volatile ("sti; nop; cli");
		CYCLES(t1);
		sched_yield();
		CYCLES(t2);
		if (i > 10) {
			if (t2 - t1 < min)
				min = t2 - t1;
		}
	}
	asm volatile ("sti");

	kill(pid, 9);
	printf("scheduling latency: %ld cycles\n", min);
	sched_yield();
}

int main(void)
{
	struct sched_param p = { sched_priority: 2 };
	unsigned long mask = 1;

	if (iopl(3)) {
		printf("need to run as root!\n");
		exit(-1);
	}
	sched_setscheduler(0, SCHED_FIFO, &p);
	sched_set_affinity(0, sizeof(mask), &mask);

	printf("default no ioperm:             ");
	measure();

	printf("turning on port 80 ioperm:     ");
	ioperm(0x80,1,1);
	measure();

	printf("turning on port 65535 ioperm:  ");
	if (ioperm(0xffff,1,1))
		printf("FAILED - older kernel.\n");
	else
		measure();

	return 0;
}

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

0 files changed, 0 insertions, 0 deletions