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>
