On 28 October 2015 at 18:21, Leonid Yegoshin <leonid.yegos...@imgtec.com> wrote: > > On 10/28/2015 03:20 AM, Alex Smith wrote: >> >> On 27 October 2015 at 20:46, Leonid Yegoshin <leonid.yegos...@imgtec.com> >> wrote: >>> >>> I believe, until this issue is fixed the R4K only CPU should be excluded >>> from VDSO timing acceleration. >> >> The VDSO code will currently use the CP0 count whenever the kernel is >> using it as its primary clocksource, aside from the case where RDHWR >> is broken as it is on old QEMUs. > > > 1) I don't see that in code - there is no check that kernel uses actually > uses R4K clocksource as primary (A), and if kernel uses R4K count as a > clocksource and later switches to some more precise clocksource then there is > no change in VDSO gettimeofday handling (B).
Incorrect. The vdso_clock_mode flag in arch_clocksource_data that I mentioned in my previous email is copied into the VDSO data page by update_vsyscall(), which is called when the clocksource changes. > > 2) The fact that MIPS kernel as today uses CP0_COUNT in any core as the same > clocksource is correct only until first power saving event with CPU clock > disabled (skipping Octeon). After that it is an incorrect use without an > accurate synchronization and that synchronization doesn't exist. > > And I remember that today kernel uses only CPU0 CP0_COUNT to update time... > may be wrong, need to check, but that could be a good code. > >> >> Maybe I'm missing something but I don't see anything in the generic >> timekeeping code that handles the same clocksource being >> unsynchronised or running at a different rate on different CPUs. > > > (I would like to skip here the generic timekeeping code capabilities, just to > restrict a discussion to HW capabilities) > >> >> Given that, if you think there is an issue that prevents the VDSO from >> using it then that would surely affect the kernel as well and needs to >> be fixed separately? >> >> If it really is necessary to prevent the VDSO from using a certain >> clocksource even though the kernel is using it, it should be a simple >> matter of setting clocksource.archdata.vdso_clock_mode to >> VDSO_CLOCK_NONE. This is how this patch stops it using the CP0 count >> when RDHWR is broken. > > > OK, just put kernel-build time check that it is not SMP without GIC > clocksource or it is Octeon. It would be enough to stop a mess. If you feel it's necessary then please do. Thanks, Alex > > - Leonid > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
