On 04/16/2019 04:26 PM, Waiman Long wrote: > On 04/16/2019 10:12 AM, Peter Zijlstra wrote: >> On Sat, Apr 13, 2019 at 01:22:50PM -0400, Waiman Long wrote: >>> +/* >>> + * The typical HZ value is either 250 or 1000. So set the minimum waiting >>> + * time to 4ms in the wait queue before initiating the handoff protocol. >>> + */ >>> +#define RWSEM_WAIT_TIMEOUT (HZ/250) >> That seems equally unfortunate. For HZ=100 that results in 0ms, and for >> HZ=300 that results in 3 1/3-rd ms. >> >> (and this is not considering Alpha,ARM and MIPS, who all have various >> other 'creative' HZ values) >> >> In general aiming for sub 10ms timing using jiffies seems 'optimistic'. > I see your point. I will change it to use sched_clock() instead. >
Thinking about it again. I think I will just change its definition to "((HZ + 249)/250)" for now to make sure that it is at least 1. The handoff waiting period isn't as important in the overall scheme. Using sched_clock() will definitely have a higher overhead than reading jiffies. I want to minimize delay before the waiter can attempt to steal the lock in the slowpath. That is the main reason I use this simple scheme. We can certain change it later on if we choose to, but I would like to focus on other more important things first. Cheers, Longman
