On Fri, 2018-06-01 at 13:03 -0700, Andy Lutomirski wrote: > Mike, you never did say: do you have PCID on your CPU? Also, what is > your workload doing to cause so many switches back and forth between > init_mm and a task. > > The point of the optimization is that switching to init_mm() should > be > fairly fast on a PCID system, whereas an IPI to do the deferred flush > is very expensive regardless of PCID.
While I am sure that bit is true, Song and I observed about 4x as much CPU use in the atomic operations in cpumask_clear_cpu and cpumask_set_cpu (inside switch_mm_irqs_off) as we saw CPU used in the %cr3 reload itself. Given how expensive those cpumask updates are, lazy TLB mode might always be worth it, especially on larger systems. -- All Rights Reversed.
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