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Date: Mon, 28 Apr 2008 17:33:43 +0530 From: Gautham R Shenoy <ego@...ibm.com> To: Oleg Nesterov <oleg@...sign.ru> Cc: Heiko Carstens <heiko.carstens@...ibm.com>, Peter Zijlstra <peterz@...radead.org>, Johannes Berg <johannes@...solutions.net>, Martin Schwidefsky <schwidefsky@...ibm.com>, Srivatsa Vaddagiri <vatsa@...ibm.com>, linux-kernel@...r.kernel.org Subject: Re: get_online_cpus() && workqueues On Mon, Apr 28, 2008 at 02:56:49PM +0400, Oleg Nesterov wrote: > On 04/28, Gautham R Shenoy wrote: > > > > On Sat, Apr 26, 2008 at 06:43:30PM +0400, Oleg Nesterov wrote: > > > > > > Can't we add another nested lock which is dropped right after __cpu_die()? > > > (in fact I think it could be dropped after __stop_machine_run). > > > > > > The new read-lock is get_online_map() (just a random name for now). The only > > > difference wrt get_online_cpus() is that it doesn't protect against CPU_DEAD, > > > but most users of get_online_cpus() doesn't need this, they only need a > > > stable cpu_online_map and sometimes they need to be sure that some per-cpu > > > object (say, cpu_workqueue_struct->thread) can't be destroyed under this > > > lock. > > > > > > get_online_map() seem to fit for this, and can be used from work->func(). > > > (actually, I think most users of use get_online_cpus() could use the new > > > helper instead, but this doen't matter). > > > > However, subsystems such as cpufreq require serialization with respect > > to the whole CPU-Hotplug operation since they do initialization and > > cleanup pre and post the change of the cpu_online_map. > > The current code, or this patch doesn't help in such cases > > when such subsystems have multithreaded workqueues! > > Yes, I see, thanks. Heiko has pointed this too. > > > One of the thoughts I have is to provide an API along the lines of > > try_get_online_cpus() which will return 1 if there is no CPU Hotplug > > operation in progress and will return 0 otherwise. In case where > > a cpu-hotplug operation is in progress, the workitem could simply > > do nothing other than requeue itself and wait for the cpu-hotplug > > operation to complete. > > Yes, possible, but it is not nice that work->func() can't just use > get_online_cpus()... Like I said, it depends on what they want to use it for. If it is just protection against the changing of the cpu_online_map then, it's simple as using get_online_map(), i.e the patch you provided. BTW, the other thing I am concerned about is the naming. Dont the names get_online_cpus() and get_online_map() appear very similar. The last thing we want is driver writers getting confused over what API to use! > > > Else, we might want to do something like what slab.c does. > > It sets the per-cpu work.func of the cpu-going down to NULL in > > CPU_DOWN_PREPARE. > > Yes, but this is different. Please note also that this particular > work must not use get_online_cpus(), no matter what changes we can > make. Otherwise cancel_delayed_work_sync() can deadlock. > > What do you think about another patch I sent? I am not happy with it, > and it certainly uglifies cpu.c, but it is simple... I am currently testing out the patchstack sent by peterz. Once that's done I will see if I can integrate this patch with the previous patches and repost the whole series. > > Oleg. -- Thanks and Regards gautham -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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