Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On 2019-08-10 01:18:34 [-0700], Christoph Hellwig wrote: > > > Does SLUB work on -rt at all? > > > > It's the only allocator we support with a few tweaks :) > > What do you do about this particular piece of code there? This part remains untouched. This "lock" is acquired within ->list_lock which is a raw_spinlock_t and disables preemption/interrupts on -RT. Sebastian
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On Sat, Aug 10, 2019 at 01:18:34AM -0700, Christoph Hellwig wrote: > On Thu, Aug 08, 2019 at 09:54:03AM +0200, Thomas Gleixner wrote: > > > I know. But the problem here is that normally PG_locked is used together > > > with wait_on_page_bit_*, but this one instances uses the bit spinlock > > > helpers. This is the equivalent of calling spin_lock on a struct mutex > > > rather than having a mutex_lock_spin helper for this case. > > > > Yes, I know :( > > But this means we should exclude slub from the bit_spin_lock removal. > It really should use it's own version of it anyhow insted of pretending > that the page lock is a bit spinlock. But PG_locked isn't used as a mutex _when the page is allocated by slab_. Yes, every other user uses PG_locked as a mutex, but I don't see why that should constrain slub's usage of PG_locked.
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On Thu, Aug 08, 2019 at 09:54:03AM +0200, Thomas Gleixner wrote: > > I know. But the problem here is that normally PG_locked is used together > > with wait_on_page_bit_*, but this one instances uses the bit spinlock > > helpers. This is the equivalent of calling spin_lock on a struct mutex > > rather than having a mutex_lock_spin helper for this case. > > Yes, I know :( But this means we should exclude slub from the bit_spin_lock removal. It really should use it's own version of it anyhow insted of pretending that the page lock is a bit spinlock. > > > Does SLUB work on -rt at all? > > It's the only allocator we support with a few tweaks :) What do you do about this particular piece of code there?
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On Thu, 8 Aug 2019, Christoph Hellwig wrote: > On Thu, Aug 08, 2019 at 09:02:47AM +0200, Thomas Gleixner wrote: > > > > mm/slub.c: bit_spin_lock(PG_locked, >flags); > > > > > > One caller ouf of a gazillion that spins on the page lock instead of > > > sleepign on it like everyone else. That should not have passed your > > > smell test to start with :) > > > > I surely stared at it, but that cannot sleep. It's in the middle of a > > preempt and interrupt disabled region and used on architectures which do > > not support CMPXCHG_DOUBLE and ALIGNED_STRUCT_PAGE ... > > I know. But the problem here is that normally PG_locked is used together > with wait_on_page_bit_*, but this one instances uses the bit spinlock > helpers. This is the equivalent of calling spin_lock on a struct mutex > rather than having a mutex_lock_spin helper for this case. Yes, I know :( > Does SLUB work on -rt at all? It's the only allocator we support with a few tweaks :) Thanks, tglx
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On Thu, Aug 08, 2019 at 09:02:47AM +0200, Thomas Gleixner wrote: > > > mm/slub.c: bit_spin_lock(PG_locked, >flags); > > > > One caller ouf of a gazillion that spins on the page lock instead of > > sleepign on it like everyone else. That should not have passed your > > smell test to start with :) > > I surely stared at it, but that cannot sleep. It's in the middle of a > preempt and interrupt disabled region and used on architectures which do > not support CMPXCHG_DOUBLE and ALIGNED_STRUCT_PAGE ... I know. But the problem here is that normally PG_locked is used together with wait_on_page_bit_*, but this one instances uses the bit spinlock helpers. This is the equivalent of calling spin_lock on a struct mutex rather than having a mutex_lock_spin helper for this case. Does SLUB work on -rt at all?
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On Mon, 5 Aug 2019, Christoph Hellwig wrote: > On Fri, Aug 02, 2019 at 11:07:53AM +0200, Thomas Gleixner wrote: > > Last time I did, there was resistance :) > > Do you have a pointer? Note that in the buffer head case maybe > a hash lock based on the page address is even better, as we only > ever use the lock in the first buffer head of a page anyway.. I need to search my archives, but I'm on a spotty and slow connection right now. Will do so when back home. > > What about the page lock? > > > > mm/slub.c: bit_spin_lock(PG_locked, >flags); > > One caller ouf of a gazillion that spins on the page lock instead of > sleepign on it like everyone else. That should not have passed your > smell test to start with :) I surely stared at it, but that cannot sleep. It's in the middle of a preempt and interrupt disabled region and used on architectures which do not support CMPXCHG_DOUBLE and ALIGNED_STRUCT_PAGE ... Thanks, tglx
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
On Fri, Aug 02, 2019 at 11:07:53AM +0200, Thomas Gleixner wrote: > Last time I did, there was resistance :) Do you have a pointer? Note that in the buffer head case maybe a hash lock based on the page address is even better, as we only ever use the lock in the first buffer head of a page anyway.. > What about the page lock? > > mm/slub.c: bit_spin_lock(PG_locked, >flags); One caller ouf of a gazillion that spins on the page lock instead of sleepign on it like everyone else. That should not have passed your smell test to start with :)
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
Christoph, On Fri, 2 Aug 2019, Christoph Hellwig wrote: > did you look into killing bіt spinlocks as a public API instead? Last time I did, there was resistance :) But I'm happy to try again. > The main users seems to be buffer heads, which are so bloated that > an extra spinlock doesn't really matter anyway. > > The list_bl and rhashtable uses kinda make sense to be, but they are > pretty nicely abstracted away anyway. The remaining users look > pretty questionable to start with. What about the page lock? mm/slub.c: bit_spin_lock(PG_locked, >flags); Thanks, tglx
Re: [patch V2 0/7] fs: Substitute bit-spinlocks for PREEMPT_RT and debugging
Hi Thomas, did you look into killing bіt spinlocks as a public API instead? The main users seems to be buffer heads, which are so bloated that an extra spinlock doesn't really matter anyway. The list_bl and rhashtable uses kinda make sense to be, but they are pretty nicely abstracted away anyway. The remaining users look pretty questionable to start with.
