On Fri, Feb 02, 2018 at 12:42:29PM +0100, Rafael J. Wysocki wrote: > > > > But now that you made me look, intel_pstate_hwp_set() is horrible crap. > > > > You should _never_ do things like: > > > > > > > > rdmsr_on_cpu() > > > > /* frob value */ > > > > wrmsr_on_cpu() > > > > > > > > That's insane. > > > > > > I guess you mean it does too many IPIs? Or that it shouldn't do any IPIs > > > at all? > > > > Yes, too many synchronous IPIs, which themselves are typically already > > more expensive than the MSR access. > > We could do all of the updates in one IPI (as Srinivas said), but it would be > more code, and custom code for that matter. > > Is this really worth it for a slow path like this one? >
Maybe it's a slow path at the moment but don't forget that one motivation for this series is that HWP does not properly react when utilisation of a CPU is artifically low because a task recently slept for IO or recently migrated. In both cases, the task may be busy and sensitive to either latency, throughput or both but HWP will use a low p-state. A standard driver can do io-boost and while it currently does not do so, it could also trivially do idle-boosting -- HWP does neither. -- Mel Gorman SUSE Labs
