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However, as I mentioned earlier, > >> >> > my main intention was to hear opinions specifically about memory contention. > >> >> > > >> >> > That said, if there is no memory contention, > >> >> > I don’t think using the _nolock API is necessarily a bad approach. > >> >> > >> >> > >> >> > In fact, I believe a bigger issue is that, under PREEMPT_RT, > >> >> > code that uses the regular memory allocation APIs may give users the false impression > >> >> > that those APIs are “safe to use,” even though they are not. > >> >> > >> >> Yeah, I share this concern. I would bet I have written code that's > >> >> broken under PREEMPT_RT (luckily only in Google's kernel fork). The > >> >> comment for GFP_ATOMIC says: > >> >> > >> >> * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower > >> >> * watermark is applied to allow access to "atomic reserves". > >> >> * The current implementation doesn't support NMI and few other strict > >> >> * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT. > >> >> > >> >> It kinda sounds like it's supposed to be OK to use GFP_ATOMIC in a > >> >> normal preempt_disable() context. So do you know exactly why it's > >> >> invalid to use it in this stop_machine() context here? Maybe we need to > >> >> update this comment. > >> > > >> > In non-PREEMPT_RT configurations, this is fine to use. > >> > However, in PREEMPT_RT, it should not be used because > >> > spin_lock becomes a sleepable lock backed by an rt-mutex. > >> > > >> > From Documentation/locking/locktypes.rst: > >> > > >> > The fact that PREEMPT_RT changes the lock category of spinlock_t and > >> > rwlock_t from spinning to sleeping. > >> > > >> > As you know, all locks related to memory allocation > >> > (e.g., zone_lock, PCP locks, etc.) use spin_lock, > >> > which becomes sleepable under PREEMPT_RT. > >> > > >> > The callback of stop_machine() is executed in a preemption-disabled context > >> > (see cpu_stopper_thread()). In this context, if it fails to acquire a spinlock > >> > during memory allocation, > >> > the task would be able to go to sleep while preemption is disabled, > >> > which is an obviously problematic situation. > >> > >> But this is what I mean, doesn't this sound like the GFP_ATOMIC comment > >> I quoted is wrong (or at least, it implies things which are wrong)? The > >> comment refers specifically to raw_spin_lock() and "strict > >> non-preemptive contexts". Which sounds like it is being written with > >> PREEMPT_RT in mind. But that doesn't really match what you've said. > > > > No. I think the comment of GFP_ATOMIC is right. > > It definitely said: > > The current implementation *doesn't support* NMI and few other strict > > *non-preemptive contexts (e.g. raw_spin_lock)*. > > But this phrasing sounds like there are other non-preemptive contexts > that it _does_ support. I would definitely read this as implying that > plain old preempt_disable() is OK. I don't understand what those "few > other strict contexts" are, nor why the stop_machine() context is > included in them. I think this phrasing seems to consider non-preeptive case for the priority or schedule policy but still make me confused too. But What I worth to say the stop_machine() -- exactly the callback context (stopper thread context) by stop_machine() is the same for raw_spin_lock() case where explictly disable preemption by calling preempt_disable(). The reason why raw_spin_lock() context couldn't call the GFP_ATOMIC since it explicitly disable preemption by calling preempt_disable(). stop_machine() callback context -- stopper thread's context is also the same. when it calls the callback by stopper (see cpu_stopper_thread()): ... preempt_count_inc(); ret = fn(arg); ... preempt_count_dec(); ... preemption is explicitly disabled like raw_spin_lock() So it seems to include in "few strict non-preemptive context". -- Sincerely, Yeoreum Yun