Messages in this thread Patch in this message | | | Subject | [PATCH] 2.5: conditional schedules with a preemptive kernel | From | Robert Love <> | Date | 20 Feb 2002 20:26:31 -0500 |
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Linus,
With a preemptive kernel, explicit conditional schedules when preempt_count is zero are a waste of cycles and code size. The attached patch, against 2.5.5, implements variants of cond_resched() and need_resched() that optimize away when CONFIG_PREEMPT is set.
This patch also converts some uses of cond_resched() and need_resched() to the new __cond_resched() and __need_resched() methods. These codepaths have been heavily tested and a lock is _never_ held across them. Thus we guarantee the same scheduling semantics.
Robert Love
diff -urN linux-2.5.5/Documentation/DocBook/kernel-hacking.tmpl linux/Documentation/DocBook/kernel-hacking.tmpl --- linux-2.5.5/Documentation/DocBook/kernel-hacking.tmpl Tue Feb 19 21:11:03 2002 +++ linux/Documentation/DocBook/kernel-hacking.tmpl Wed Feb 20 19:10:46 2002 @@ -371,7 +371,16 @@ </para> <programlisting> -cond_resched(); /* Will sleep */ +cond_resched(); /* Will sleep */ + </programlisting> + + <para> + or, if you know the lock count is zero (and thus the kernel is + preemptible), don't waste the cycles: + </para> + + <programlisting> +__cond_resched(); /* Is not called if CONFIG_PREEMPT is set */ </programlisting> <para> diff -urN linux-2.5.5/drivers/char/random.c linux/drivers/char/random.c --- linux-2.5.5/drivers/char/random.c Tue Feb 19 21:10:55 2002 +++ linux/drivers/char/random.c Wed Feb 20 19:35:24 2002 @@ -1308,9 +1308,10 @@ wake_up_interruptible(&random_write_wait); r->extract_count += nbytes; - + ret = 0; while (nbytes) { +#ifndef CONFIG_PREEMPT /* * Check if we need to break out or reschedule.... */ @@ -1322,6 +1323,7 @@ } schedule(); } +#endif /* Hash the pool to get the output */ tmp[0] = 0x67452301; diff -urN linux-2.5.5/drivers/char/tty_io.c linux/drivers/char/tty_io.c --- linux-2.5.5/drivers/char/tty_io.c Tue Feb 19 21:11:04 2002 +++ linux/drivers/char/tty_io.c Wed Feb 20 19:35:37 2002 @@ -712,7 +712,7 @@ ret = -ERESTARTSYS; if (signal_pending(current)) break; - cond_resched(); + __cond_resched(); } } if (written) { diff -urN linux-2.5.5/fs/namei.c linux/fs/namei.c --- linux-2.5.5/fs/namei.c Tue Feb 19 21:11:00 2002 +++ linux/fs/namei.c Wed Feb 20 19:36:01 2002 @@ -342,7 +342,7 @@ goto loop; if (current->total_link_count >= 40) goto loop; - if (need_resched()) { + if (__need_resched()) { current->state = TASK_RUNNING; schedule(); } diff -urN linux-2.5.5/include/linux/sched.h linux/include/linux/sched.h --- linux-2.5.5/include/linux/sched.h Tue Feb 19 21:10:55 2002 +++ linux/include/linux/sched.h Wed Feb 20 19:41:30 2002 @@ -830,7 +830,22 @@ { return unlikely(test_tsk_thread_flag(p,TIF_SIGPENDING)); } - + +/* + * With a preemptible kernel, explicit conditional schedules + * when preempt_count is zero are a waste of cycles. If you + * know the lock count is zero, use + * __cond_resched() instead of cond_resched() and + * __need_resched() instead of need_resched() + */ +#ifndef CONFIG_PREEMPT +#define __cond_resched() cond_resched() +#define __need_resched() need_resched() +#else +#define __cond_resched() do { } while(0) +#define __need_resched() 0 +#endif + static inline int need_resched(void) { return unlikely(test_thread_flag(TIF_NEED_RESCHED)); diff -urN linux-2.5.5/kernel/softirq.c linux/kernel/softirq.c --- linux-2.5.5/kernel/softirq.c Tue Feb 19 21:10:58 2002 +++ linux/kernel/softirq.c Wed Feb 20 18:08:09 2002 @@ -387,7 +387,7 @@ while (softirq_pending(cpu)) { do_softirq(); - cond_resched(); + __cond_resched(); } __set_current_state(TASK_INTERRUPTIBLE); diff -urN linux-2.5.5/mm/filemap.c linux/mm/filemap.c --- linux-2.5.5/mm/filemap.c Tue Feb 19 21:11:00 2002 +++ linux/mm/filemap.c Wed Feb 20 19:38:06 2002 @@ -401,7 +401,7 @@ } page_cache_release(page); - if (need_resched()) { + if (__need_resched()) { __set_current_state(TASK_RUNNING); schedule(); } diff -urN linux-2.5.5/mm/vmscan.c linux/mm/vmscan.c --- linux-2.5.5/mm/vmscan.c Tue Feb 19 21:10:56 2002 +++ linux/mm/vmscan.c Wed Feb 20 19:07:58 2002 @@ -627,7 +627,7 @@ for (i = pgdat->nr_zones-1; i >= 0; i--) { zone = pgdat->node_zones + i; - cond_resched(); + __cond_resched(); if (!zone->need_balance) continue; if (!try_to_free_pages(zone, GFP_KSWAPD, 0)) {
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