• Milton Miller's avatar
    kernel/smp.c: consolidate writes in smp_call_function_interrupt() · 225c8e01
    Milton Miller authored
    We have to test the cpu mask in the interrupt handler before checking the
    refs, otherwise we can start to follow an entry before its deleted and
    find it partially initailzed for the next trip.  Presently we also clear
    the cpumask bit before executing the called function, which implies
    getting write access to the line.  After the function is called we then
    decrement refs, and if they go to zero we then unlock the structure.
    
    However, this implies getting write access to the call function data
    before and after another the function is called.  If we can assert that no
    smp_call_function execution function is allowed to enable interrupts, then
    we can move both writes to after the function is called, hopfully allowing
    both writes with one cache line bounce.
    
    On a 256 thread system with a kernel compiled for 1024 threads, the time
    to execute testcase in the "smp_call_function_many race" changelog was
    reduced by about 30-40ms out of about 545 ms.
    
    I decided to keep this as WARN because its now a buggy function, even
    though the stack trace is of no value -- a simple printk would give us the
    information needed.
    
    Raw data:
    
    Without patch:
      ipi_test startup took 1219366ns complete 539819014ns total 541038380ns
      ipi_test startup took 1695754ns complete 543439872ns total 545135626ns
      ipi_test startup took 7513568ns complete 539606362ns total 547119930ns
      ipi_test startup took 13304064ns complete 533898562ns total 547202626ns
      ipi_test startup took 8668192ns complete 544264074ns total 552932266ns
      ipi_test startup took 4977626ns complete 548862684ns total 553840310ns
      ipi_test startup took 2144486ns complete 541292318ns total 543436804ns
      ipi_test startup took 21245824ns complete 530280180ns total 551526004ns
    
    With patch:
      ipi_test startup took 5961748ns complete 500859628ns total 506821376ns
      ipi_test startup took 8975996ns complete 495098924ns total 504074920ns
      ipi_test startup took 19797750ns complete 492204740ns total 512002490ns
      ipi_test startup took 14824796ns complete 487495878ns total 502320674ns
      ipi_test startup took 11514882ns complete 494439372ns total 505954254ns
      ipi_test startup took 8288084ns complete 502570774ns total 510858858ns
      ipi_test startup took 6789954ns complete 493388112ns total 500178066ns
    
    	#include <linux/module.h>
    	#include <linux/init.h>
    	#include <linux/sched.h> /* sched clock */
    
    	#define ITERATIONS 100
    
    	static void do_nothing_ipi(void *dummy)
    	{
    	}
    
    	static void do_ipis(struct work_struct *dummy)
    	{
    		int i;
    
    		for (i = 0; i < ITERATIONS; i++)
    			smp_call_function(do_nothing_ipi, NULL, 1);
    
    		printk(KERN_DEBUG "cpu %d finished\n", smp_processor_id());
    	}
    
    	static struct work_struct work[NR_CPUS];
    
    	static int __init testcase_init(void)
    	{
    		int cpu;
    		u64 start, started, done;
    
    		start = local_clock();
    		for_each_online_cpu(cpu) {
    			INIT_WORK(&work[cpu], do_ipis);
    			schedule_work_on(cpu, &work[cpu]);
    		}
    		started = local_clock();
    		for_each_online_cpu(cpu)
    			flush_work(&work[cpu]);
    		done = local_clock();
    		pr_info("ipi_test startup took %lldns complete %lldns total %lldns\n",
    			started-start, done-started, done-start);
    
    		return 0;
    	}
    
    	static void __exit testcase_exit(void)
    	{
    	}
    
    	module_init(testcase_init)
    	module_exit(testcase_exit)
    	MODULE_LICENSE("GPL");
    	MODULE_AUTHOR("Anton Blanchard");
    Signed-off-by: default avatarMilton Miller <miltonm@bga.com>
    Cc: Anton Blanchard <anton@samba.org>
    Cc: Ingo Molnar <mingo@elte.hu>
    Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    225c8e01
smp.c 15.1 KB