On Wed, Dec 3, 2025 at 11:37 AM Benjamin Marzinski <[email protected]> wrote: > > On Thu, Nov 27, 2025 at 12:31:12PM +0100, Martin Wilck wrote: > > On Mon, 2025-11-24 at 15:11 -0500, Benjamin Marzinski wrote: > > > On Fri, Nov 21, 2025 at 02:35:23PM -0800, Brian Bunker wrote: > > > > > > > > > > > I added the purge thread because I didn't want to starve the > > > > checker thread at a large disconnect volume scale. I noticed > > > > that the number of devices if I purged inline with the check that > > > > it didn't scale linearly after a point and seemed to be > > > > significantly > > > > starving the checker thread. Doing the purge in another thread > > > > seemed to relieve that pressure. > > > > > > That might be because the check thread has safeguards to try to avoid > > > starving the other threads. Since a lot of multipathd's work is gated > > > by > > > the vecs lock, there's only so much parallelism that can happen with > > > multiple threads. If deleting the devices is taking a long time, it > > > might > > > be better for this to get interrupted, so that other threads can run. > > > > > > Since purging the paths is fairly low priority, we could continue to > > > run > > > it in its own thread, but instead of running through all the devices > > > at > > > once, purgeloop could lock the vecs->lock, handle one device, and > > > then > > > unlock and loop. This means it would need to search through the list > > > from the start each time it regrabbed the lock, since the list could > > > have changed while it wasn't holding it. When purgeloop makes it all > > > the > > > way through the list without finding any paths that are due for a > > > delete > > > attempt, it can sleep or block waiting for more paths. > > > > > > This would mean that paths would need to remember if they had already > > > been handled in a cycle. That could be done by purgeloop keeping > > > track > > > of which cycle it was on, and the paths storing the cycle number > > > whenever they were checked. > > > > As I wrote in my other post, wish that this thread wouldn't hold the > > vecs lock at all. multipathd could simply use a pipe to write dev_t's > > of to-be-purged paths to the purge thread (or process :-) ). > > This seems like a good idea. Obviously, writing to sysfs while the vecs > lock is being held means that multipath will continue to have that scsi > device open, so there's no chance of the device name getting reused if > the device is removed from the system. But this other thread/process > could simply open the scsi device, double check that it's still > disconnected, write to sysfs, and then close the device. That would > offer the same guarantee, without the need to interact with anything > else. > > > Martin > Martin, If you look at the v3 implementation that I sent up yesterday, you will see we are no longer holding the vecs lock during the sysfs delete operation. We also made the delete operation non-blocking.
The two places in the purgeloop where we do hold the lock are for two very short operations: 1. We scan the vector to find one path that needs purging. We update pp->purge_cycle and increment the retry counter pp->purge_retries++. Then we copy the necessary data for the purge into the purge_info data structure. Then we release the lock. There shouldn't be much contention here with these operations. 2. We also re-acquire the lock to make sure the path still exists in the pathvec and clear the purge_path and reset purge_retries. This should also be very fast. We can't take the approach to open the device since it is disconnected. I think it will just fail or hang. We have the udev reference to make sure the device object is alive and not freed underneath us. Thanks, Brian -- Brian Bunker PURE Storage, Inc. [email protected]
