On Wed, 4 Mar 2026 23:30:40 -0500 Waiman Long <[email protected]> wrote:
> On 3/4/26 10:08 PM, Yafang Shao wrote: > > On Thu, Mar 5, 2026 at 11:00 AM Steven Rostedt <[email protected]> wrote: > > > >> On Thu, 5 Mar 2026 10:33:00 +0800 > >> Yafang Shao <[email protected]> wrote: > >> > >>> Other tools may also read available_filter_functions, requiring each > >>> one to be patched individually to avoid this flaw—a clearly > >>> impractical solution. > >> What exactly is the issue? > > It makes no sense to spin unnecessarily when it can be avoided. We > > continuously improve the kernel to do the right thing—and unnecessary > > spinning is certainly not the right thing. > > > >> If a task does a while 1 in user space, it > >> wouldn't be much different. > > The while loop in user space performs actual work, whereas useless > > spinning does nothing but burn CPU cycles. My point is simple: if this > > unnecessary spinning isn't already considered an issue, it should > > be—it's something that clearly needs improvement. > > The whole point of optimistic spinning is to reduce the lock acquisition > latency. If the waiter sleeps, the unlock operation will have to wake up > the waiter which can have a variable latency depending on how busy the > system is at the time. Yes, it is burning CPU cycles while spinning, > Most workloads will gain performance with this optimistic spinning > feature. You do have a point that for system monitoring tools that > observe the system behavior, they shouldn't burn that much CPU times > that affect performance of real workload that the tools are monitoring. > > BTW, you should expand the commit log of patch 1 to include the > rationale of why we should add this feature to mutex as the information > in the cover letter won't get included in the git log if this patch > series is merged. You should also elaborate in comment on under what > conditions should this this new mutex API be used. Isn't changing mutex_lock() the wrong place anyway? What you need is for the code holding the lock to indicate that it isn't worth waiters spinning because the lock will be held for a long time. David > > Cheers, > Longman >
