From: Martin Pieuchot Subject: Re: uvm_purge() To: Mark Kettenis Cc: claudio@openbsd.org, tech@openbsd.org Date: Wed, 21 May 2025 08:51:11 +0200 On 20/05/25(Tue) 22:04, Mark Kettenis wrote: > > Date: Fri, 16 May 2025 15:58:41 +0200 > > From: Martin Pieuchot > > > > On 15/05/25(Thu) 14:45, Mark Kettenis wrote: > > > > Date: Thu, 15 May 2025 12:34:16 +0200 > > > > From: Martin Pieuchot > > > > > > > > On 15/05/25(Thu) 11:52, Mark Kettenis wrote: > > > > > > Date: Wed, 14 May 2025 11:55:32 +0200 > > > > > > From: Martin Pieuchot > > > > > > > > > > Hi Martin, > > > > > > > > > > Sorry, I'm a bit slow. Have concerts coming up so my OpenBSD time is > > > > > a bit limited at the moment. > > > > > > > > No worries, thanks for your answer! > > > > > > > > > [...] > > > > > > > It is important that when we flush the TLB, none of the threads in a > > > > > > > process have the userland page tables active. On arm64 the CPUs can > > > > > > > speculatively load TLB entries even if you don't reference the pages! > > > > > > > The current code deactivates the page tables in cpu_exit() and uses > > > > > > > atomics to make sure that the last thread that goes through cpu_exit() > > > > > > > also flushes the TLB. At that point none of the threads can sleep, so > > > > > > > we can simply set the TTBR0_EL1 register to point at a page filled > > > > > > > with zeroes and don't have to worry about a context switch resetting > > > > > > > TTBR0_EL1 to point at the userland page tables again. (We re-use the > > > > > > > page filled with zeroes from the kernel pmap for that.) > > > > > > > > > > > > > > But uvm_purge() can sleep, so it needs to be called much earlier. We > > > > > > > can prevent a context switch from reloading TTBR0_EL1 by also setting > > > > > > > pm->pm_pt0a to point at that page filled with zeroes. But if we do > > > > > > > that for any non-main thread, we run into problems because another > > > > > > > thread that is still running userland code might context switch and > > > > > > > end up in an endless loop faulting because it has a page table without > > > > > > > valied entries in it. > > > > > > > > > > > > > > So that's why my new pmap_exit() function gets called in different > > > > > > > places for the main thread and other threads. The main thread calls > > > > > > > pmap_exit() at the place where your diff calls uvm_purge(), so it > > > > > > > could be rolled into that function. > > > > > > > > > > > > > > I think this strategy will work for other pmaps as well, but I > > > > > > > probably should look at one or two other ones. > > > > > > > > > > > > uvm_purge() is executed by the last thread in a process. When this > > > > > > happens the other threads might still be at the end of exit1() but none > > > > > > of them will go back to userland. > > > > > > > > > > > > I have other diffs to improve the synchronization between the siblings > > > > > > of a process when exiting, mostly to remove unnecessary context switches. > > > > > > They are built on the current assumption that uvm_purge() is called when > > > > > > all other threads have cleaned their states. > > > > > > This part of my work removes the notion of 'main thread' and the > > > > > > P_THREAD flag. Instead the last thread of a process to enter exit1() > > > > > > will clean the per-process states. > > > > > > > > > > > > Could you use those two pieces of information to simplify your diff? > > > > > > > > > > This suggests that I really should have two seperate functions, one > > > > > which gets called for each thread exit (which disables the userland > > > > > page tables) and one that gets called from uvm_purge() (which does the > > > > > TLB flush and can clean up the pmap in the future). That way I don't > > > > > have to rely on P_THREAD to determine what to do. > > > > > > > > I agree. > > > > > > > > > The latter function should probably be called pmap_purge() and it is > > > > > fine if we call it for the "last thread in the process" instead of > > > > > what we currently consider the "main thread". But this function still > > > > > needs to make sure it runs after the other threads have disabled their > > > > > userland page tables. And as you point out, at the point where > > > > > uvm_purge() gets called the other threads might still be at the tail > > > > > end of exit1(). > > > > > > > > > > I'm a bit hesitant to add yet another "public" pmap interface, so it > > > > > would be nice to have cpu_exit() handle the MD-specifics of disabling > > > > > the userland page tables. I could put back the atomics to manipulate > > > > > pm_active and keep that code in pmap_deactivate(). The ordering issue > > > > > between calling uvm_purge() and the other threads going through > > > > > cpu_exit() is largely theoretical as there isn't much code between the > > > > > wakup(&pr->ps_threads) and the cpu_exit() call. And I could make > > > > > pmap_purge() block until pm_active becomes 1 to make sure the other > > > > > threads have gone through cpu_exit(). > > > > > > > > I agree being able to use pmap_deactivate() is cleaner. > > > > > > > > I'd like to avoid adding another barrier in exit1(). I'm actually > > > > working hard to remove them as much as possible to reduce existing > > > > latency. > > > > > > The added "barrier" in pmap_purge() would be something like: > > > > > > while (pm->pm_active != 1) > > > CPU_BUSY_CYCLE; > > > > > > And I don't think we'd execute that loop under normal conditions. We > > > could put that in as a temporary measure until the rest of this is > > > figured out to make sure arm64 isn't blocking your progress. We can > > > replace the while loop with a KASSERT when the time comes to do that. > > > > Fine with me. > > > > > Alternatively we could skip the pmap_purge() optimization if the other > > > threads haven't gone through cpu_exit() yet. > > > > > > > I can send my other diff with the signaling to wake up the last thread > > > > after cpu_exit(). This will require some plumbing to move sched_exit() > > > > out of cpu_exit(). > > > > > > Yes, I think moving the sched_exit() out of cpu_exit() and into > > > exit1() would make sense. I wonder if we should try to move > > > pmap_deactivate() into exit1() as well? Almost all cpu_exit() > > > implementations call it just before sched_exit(). > > > > Moving pmap_deactivate() as well makes sense to me. I'll do it. > > > > > Anyway, diff below is something I'm happier with. > > > > Running with it as well. > > > > ok mpi@ > > I've added a comment explaining the (need for a) barrier. If there > are no objections to introducing yet another pmap interface, I'll go > ahead with this. I believe that's what we want. ok mpi@ > Index: arch/arm64/arm64/pmap.c > =================================================================== > RCS file: /cvs/src/sys/arch/arm64/arm64/pmap.c,v > diff -u -p -r1.111 pmap.c > --- arch/arm64/arm64/pmap.c 14 Feb 2025 18:36:04 -0000 1.111 > +++ arch/arm64/arm64/pmap.c 20 May 2025 20:03:10 -0000 > @@ -1497,14 +1497,39 @@ pmap_deactivate(struct proc *p) > > KASSERT(p == curproc); > > + if (pm->pm_active == 0) > + return; > + > WRITE_SPECIALREG(ttbr0_el1, pmap_kernel()->pm_pt0pa); > __asm volatile("isb"); > > - if (atomic_dec_int_nv(&pm->pm_active) > 0) > - return; > + atomic_dec_int(&pm->pm_active); > +} > + > +void > +pmap_purge(struct proc *p) > +{ > + pmap_t pm = p->p_vmspace->vm_map.pmap; > + > + KASSERT(p->p_p->ps_threadcnt == 0); > + KASSERT(p == curproc); > + > + /* > + * There is a theoretical chance that our sibling threads are > + * still making their way through the tail end of exit1(). > + * Make absolutely sure they have made it past the point where > + * they disable their userland page tables. > + */ > + while (pm->pm_active != 1) > + CPU_BUSY_CYCLE(); > + > + WRITE_SPECIALREG(ttbr0_el1, pmap_kernel()->pm_pt0pa); > + __asm volatile("isb"); > > cpu_tlb_flush_asid_all((uint64_t)pm->pm_asid << 48); > cpu_tlb_flush_asid_all((uint64_t)(pm->pm_asid | ASID_USER) << 48); > + pm->pm_pt0pa = pmap_kernel()->pm_pt0pa; > + pm->pm_active = 0; > } > > /* > Index: arch/arm64/include/pmap.h > =================================================================== > RCS file: /cvs/src/sys/arch/arm64/include/pmap.h,v > diff -u -p -r1.28 pmap.h > --- arch/arm64/include/pmap.h 3 Feb 2025 17:59:40 -0000 1.28 > +++ arch/arm64/include/pmap.h 20 May 2025 20:03:10 -0000 > @@ -124,6 +124,7 @@ int pmap_fault_fixup(pmap_t, vaddr_t, vm > > #define __HAVE_PMAP_MPSAFE_ENTER_COW > #define __HAVE_PMAP_POPULATE > +#define __HAVE_PMAP_PURGE > > #endif /* _KERNEL && !_LOCORE */ >