Re: Additional sampling fun

speck for mark gross <[email protected]> writes:
> On Fri, Feb 28, 2020 at 05:34:47PM +0100, speck for Greg KH wrote:
>> On Fri, Feb 28, 2020 at 05:21:40PM +0100, speck for Borislav Petkov wrote:
>> Ugh.  I think we need to drag Jason into this as well, but really,
>> talking about that can be done on the mailing list as there's nothing
>> wrong with trying to get that slow code out of the irq path today,
>> right?
>> 
> FWIW unless someone is abusing rdrand/rdseed I don't think the impact of the
> mitigation will be measurable.  Running multiple instances of spanking rdrand
> in a loop will show nonlinear impacts due to bus lock contention but, I don't
> think there is any contention issues with once/64IRS's or once a second.  you
> are looking at approximately O(100cycles) vrs O(1000cycles) every second or
> every 64th interrupt.  I don't think you'll be able to measure the impact of
> that.  (unless you force lock contention on the HW bus lock)

Have several cores with a 10k+ interrupts per second and if you're
unlucky they start to contend, then the every 64th interrupt will be
measurable quite prominent.

But I agree with Greg, that we can tackle this on LKML without
mentioning that particular issue.

Thanks,

        tglx