From mboxrd@z Thu Jan 1 00:00:00 1970 From: grygorii.strashko@ti.com (Grygorii Strashko) Date: Tue, 8 Sep 2015 11:21:53 +0300 Subject: [PATCH v2 7/7] ARM: smp: Add runtime PM support for CPU hotplug In-Reply-To: <2355864.I8qZFusWfm@vostro.rjw.lan> References: <2657565.lxWtPmdjyp@vostro.rjw.lan> <55ED9328.2010501@ti.com> <2355864.I8qZFusWfm@vostro.rjw.lan> Message-ID: <55EE9AA1.8020200@ti.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On 09/07/2015 11:42 PM, Rafael J. Wysocki wrote: > On Monday, September 07, 2015 04:37:44 PM Grygorii Strashko wrote: >> On 09/07/2015 04:04 PM, Rafael J. Wysocki wrote: >>> 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. >> >> Not exactly true :(, since spinlock is converted to [rt_] mutex. >> Usually, this difference can't be seen because on -RT kernel all or >> mostly all HW IRQ handlers will be forced to be threaded. > > Exactly. And that's what I'm talking about. > >> For the cases, where such automatic conversion is not working, >> (like chained irq handlers or HW-handler+Threaded handler) the code >> has to be carefully patched to work properly as for non-RT as for -RT. > > Right. > >> Also, this triggers some -RT incompatibility issues, like with PM runtime or > > That I'm not sure about. Why would runtime PM cause problems with -RT (apart > from attempts to use it from the idle loop, but that's not happening in the > mainline anyway)? I have to be more specific - sorry. "irq_safe" mode of PM runtime is incompatible with -RT. Here is an example: - HW IRQ handler in TI OMAP GPIO driver is implemented as chained IRQ handler and contains pm_runtime_get_sync()/pm_runtime_put(). This works properly with vanilla kernel because OMAP GPIO devices marked as irq_safe. Chained IRQ handlers can't be forced threaded and PM runtime APIs trigger "sleeping function called from invalid context" issues there, so corresponding code has to be reworked. ... -- regards, -grygorii