On 5/28/24 9:56 AM, Sebastian Andrzej Siewior wrote: > On 2024-05-27 22:16:41 [-0700], Shakeel Butt wrote: >> On Mon, May 27, 2024 at 06:34:24PM GMT, Vlastimil Babka (SUSE) wrote: >> > On 5/27/24 5:22 PM, Sebastian Andrzej Siewior wrote: >> > > On 2024-04-20 16:25:05 [-0700], Shakeel Butt wrote: >> > >> mod_memcg_lruvec_state() is never called from outside of memcontrol.c >> > >> and with always irq disabled. So, replace it with the irq disabled >> > >> version and add an assert that irq is disabled in the caller. >> > > >> > > unless PREEMPT_RT is enabled. In that case IRQs are not disabled as part >> > > of local_lock_irqsave(&memcg_stock.stock_lock, …) leading to: >> >> Sorry about that and thanks for the report. > > no worries. > >> > >> > But then the "interrupts are handled by a kernel thread that can sleep" part >> > of RT also means it's ok to just have the stock_lock taken with no >> > interrupts disabled as no actual raw interrupt handler will interrupt the >> > holder and deadlock, right? > > I *don't* know why the interrupts-disabled check is here. The > memcg_stock.stock_lock is acquired on RT with interrupts enabled and > never disables interrupts. The lock is never acquired in an hard > interrupt (not threaded interrupt) context so there is never a deadlock. > > Originally the interrupts were disabled in mod_memcg_lruvec_state() > because the counter, it operates on, is per-CPU and relies on disabled > interrupts because the operation is not atomic and the code can be run > in interrupts context (on !RT). The __mod_memcg_lruvec_state() variant > of it relied on interrupts being disabled by the caller. This "rely on" > was part of a spinlock_t lock (or invoked from an interrupt handler, the > memory is fading slowly away) which does not disable interrupts on > PREEMPT_RT. > So for that reason we ended up with __memcg_stats_lock() which disables > preemption only on PREEMPT_RT to achieve the same level of "atomic" > update. > >> Thanks Vlastimil for jolting my memory on RT reasoning. >> >> > > suggestions? >> > >> > So in that case the appropriate thing would be to replace the assert with >> > lockdep_assert_held(&memcg_stock.stock_lock); >> > ? >> > >> > It seems all the code paths leading here have that one. >> > >> >> Yeah this seems right and reasonable. Should I send a fix or you want to >> send it? > > I don't mind sending a patch. I'm just not sure if the lock is the right > thing to do. However it should ensure that interrupts are disabled on > !RT for the sake of the counter update (if observed in IRQ context). Looks like some places there use VM_WARN_ON_IRQS_ENABLED() that's turned off for PREEMPT_RT, so maybe that's what should replace the current lockdep_assert, perhaps together with lockdep_assert_held(this_cpu_ptr(&memcg_stock.stock_lock)); But also __mod_memcg_lruvec_state() already has that VM_WARN_ON. > Yeah, let me prepare something. > > Sebastian
