Author: nyan Date: Tue Oct 4 13:24:22 2011 New Revision: 225977 URL: http://svn.freebsd.org/changeset/base/225977
Log: MFi386: revision 225936 Add some improvements in the idle table callbacks: - Replace instances of manual assembly instruction "hlt" call with halt() function calling. - In cpu_idle_mwait() avoid races in check to sched_runnable() using the same pattern used in cpu_idle_hlt() with the 'hlt' instruction. - Add comments explaining the logic behind the pattern used in cpu_idle_hlt() and other idle callbacks. Modified: head/sys/pc98/pc98/machdep.c Modified: head/sys/pc98/pc98/machdep.c ============================================================================== --- head/sys/pc98/pc98/machdep.c Tue Oct 4 13:19:21 2011 (r225976) +++ head/sys/pc98/pc98/machdep.c Tue Oct 4 13:24:22 2011 (r225977) @@ -1117,7 +1117,7 @@ void cpu_halt(void) { for (;;) - __asm__ ("hlt"); + halt(); } static int idle_mwait = 1; /* Use MONITOR/MWAIT for short idle. */ @@ -1136,9 +1136,22 @@ cpu_idle_hlt(int busy) state = (int *)PCPU_PTR(monitorbuf); *state = STATE_SLEEPING; + /* - * We must absolutely guarentee that hlt is the next instruction - * after sti or we introduce a timing window. + * Since we may be in a critical section from cpu_idle(), if + * an interrupt fires during that critical section we may have + * a pending preemption. If the CPU halts, then that thread + * may not execute until a later interrupt awakens the CPU. + * To handle this race, check for a runnable thread after + * disabling interrupts and immediately return if one is + * found. Also, we must absolutely guarentee that hlt is + * the next instruction after sti. This ensures that any + * interrupt that fires after the call to disable_intr() will + * immediately awaken the CPU from hlt. Finally, please note + * that on x86 this works fine because of interrupts enabled only + * after the instruction following sti takes place, while IF is set + * to 1 immediately, allowing hlt instruction to acknowledge the + * interrupt. */ disable_intr(); if (sched_runnable()) @@ -1164,11 +1177,19 @@ cpu_idle_mwait(int busy) state = (int *)PCPU_PTR(monitorbuf); *state = STATE_MWAIT; - if (!sched_runnable()) { - cpu_monitor(state, 0, 0); - if (*state == STATE_MWAIT) - cpu_mwait(0, MWAIT_C1); + + /* See comments in cpu_idle_hlt(). */ + disable_intr(); + if (sched_runnable()) { + enable_intr(); + *state = STATE_RUNNING; + return; } + cpu_monitor(state, 0, 0); + if (*state == STATE_MWAIT) + __asm __volatile("sti; mwait" : : "a" (MWAIT_C1), "c" (0)); + else + enable_intr(); *state = STATE_RUNNING; } @@ -1180,6 +1201,12 @@ cpu_idle_spin(int busy) state = (int *)PCPU_PTR(monitorbuf); *state = STATE_RUNNING; + + /* + * The sched_runnable() call is racy but as long as there is + * a loop missing it one time will have just a little impact if any + * (and it is much better than missing the check at all). + */ for (i = 0; i < 1000; i++) { if (sched_runnable()) return; _______________________________________________ svn-src-head@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-head To unsubscribe, send any mail to "svn-src-head-unsubscr...@freebsd.org"