On Mon, 4 May 2015 10:48:44 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > I'm not sure why it does the ndelay() and not just a cpu_relax(), but > > Because the code was writtenlong before cpu_relax() existed, just > like it was written long before the generic percpu counter code was > added... Ah, legacy code. > > .... > > > Now, when PREEMPT_RT is not enabled, that spin_lock() disables > > preemption. But for PREEMPT_RT, it does not. Although with my test box I > > was not able to produce a task state of all tasks, but I'm assuming that > > some task called the xfs_icsb_lock_all_counters() and was preempted by > > an RT task and could not finish, causing all callers of that lock to > > block indefinitely. > > > > Looking at all users of xfs_icsb_lock_all_counters(), they are leaf > > functions and do not call anything that may block on PREEMPT_RT. I > > believe the proper fix here is to simply disable preemption in > > xfs_icsb_lock_all_counters() when PREEMPT_RT is enabled. > > RT is going to have other performance problems that are probably > going to negate the scalability this code provides. If you want a > hack that you can easily backport (as this code now uses the generic > percpu counters) then have a look at fs/xfs/xfs_linux.h: > > /* > * Feature macros (disable/enable) > */ > #ifdef CONFIG_SMP > #define HAVE_PERCPU_SB /* per cpu superblock counters are a 2.6 feature */ > #else > #undef HAVE_PERCPU_SB /* per cpu superblock counters are a 2.6 feature */ > #endif > > You can turn off all that per-cpu code simply by: > > -#ifdef CONFIG_SMP > +#if defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_RT) Oh, I think I like this the best. Thanks for the advice. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
