From mboxrd@z Thu Jan 1 00:00:00 1970 From: rjw@rjwysocki.net (Rafael J. Wysocki) Date: Mon, 07 Sep 2015 15:04:10 +0200 Subject: [PATCH v2 7/7] ARM: smp: Add runtime PM support for CPU hotplug In-Reply-To: References: Message-ID: <2657565.lxWtPmdjyp@vostro.rjw.lan> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Saturday, September 05, 2015 11:39:20 AM Alan Stern wrote: > On Sat, 5 Sep 2015, Grygorii Strashko wrote: > > > On 09/04/2015 09:45 PM, Alan Stern wrote: > > > On Fri, 4 Sep 2015, Grygorii Strashko wrote: > > > > > >> There is one "small" problem with such approach :( > > >> > > >> - It's incompatible with -RT kernel, because PM runtime can't be used > > >> in atomic context on -RT. > > > > > > Can you explain this more fully? Why can't runtime PM be used in > > > atomic context in the -rt kernels? > > > > > > > See: > > http://lwn.net/Articles/146861/ > > https://rt.wiki.kernel.org/index.php/Frequently_Asked_Questions#How_does_the_CONFIG_PREEMPT_RT_patch_work.3F > > > > spinlock_t > > Critical sections are preemptible. The _irq operations (e.g., spin_lock_irqsave()) > > do -not- disable hardware interrupts. Priority inheritance is used to prevent priority > > inversion. An underlying rt_mutex is used to implement spinlock_t in PREEMPT_RT. > > > > As result, have to do things like: > > https://lkml.org/lkml/2015/8/18/161 > > https://lkml.org/lkml/2015/8/18/162 > > > > Sorry for brief reply - Friday/Sat night :) > > I see. Although we normally think of interrupt contexts as being > atomic, in an -rt kernel this isn't true any more because things like > spin_lock_irq don't actually disable interrupts. > > Therefore it would be correct to say that in -rt kernels, runtime PM > can be used in interrupt context (if the device is marked as irq-safe), > but not in atomic context. Right? Right. Whatever is suitable for interrupt context in the mainline, will be suitable for that in -rt kernels too. However, what is suitable for the idle loop in the mainline, may not be suitable for that in -rt kernels. That's why raw_spin_lock/unlock() need to be used within the idle loop. Thanks, Rafael