Chris Packham <Chris.Packham@xxxxxxxxxxxxxxxxxxx> writes: > On 8/11/23 12:08, Ankur Arora wrote: >> There are broadly three sets of uses of cond_resched(): >> >> 1. Calls to cond_resched() out of the goodness of our heart, >> otherwise known as avoiding lockup splats. >> >> 2. Open coded variants of cond_resched_lock() which call >> cond_resched(). >> >> 3. Retry or error handling loops, where cond_resched() is used as a >> quick alternative to spinning in a tight-loop. >> >> When running under a full preemption model, the cond_resched() reduces >> to a NOP (not even a barrier) so removing it obviously cannot matter. >> >> But considering only voluntary preemption models (for say code that >> has been mostly tested under those), for set-1 and set-2 the >> scheduler can now preempt kernel tasks running beyond their time >> quanta anywhere they are preemptible() [1]. Which removes any need >> for these explicitly placed scheduling points. >> >> The cond_resched() calls in set-3 are a little more difficult. >> To start with, given it's NOP character under full preemption, it >> never actually saved us from a tight loop. >> With voluntary preemption, it's not a NOP, but it might as well be -- >> for most workloads the scheduler does not have an interminable supply >> of runnable tasks on the runqueue. >> >> So, cond_resched() is useful to not get softlockup splats, but not >> terribly good for error handling. Ideally, these should be replaced >> with some kind of timed or event wait. >> For now we use cond_resched_stall(), which tries to schedule if >> possible, and executes a cpu_relax() if not. >> >> The cond_resched() calls here are all kinds. Those from set-1 >> or set-2 are quite straight-forward to handle. >> >> There are quite a few from set-3, where as noted above, we >> use cond_resched() as if it were a amulent. Which I supppose >> it is, in that it wards off softlockup or RCU splats. >> >> Those are now cond_resched_stall(), but in most cases, given >> that the timeouts are in milliseconds, they could be easily >> timed waits. > > For i2c-mpc.c: > > It looks as the code in question could probably be converted to > readb_poll_timeout(). If I find sufficient round-tuits I might look at > that. Regardless in the context of the tree-wide change ... > > Reviewed-by: Chris Packham <chris.packham@xxxxxxxxxxxxxxxxxxx> Thanks Chris. This'll take a while before this lands. I'll see if I can send a patch with cond_resched_stall() or similar separately. Meanwhile please feel free to make the readb_poll_timeout() change. -- ankur
