search for: c35b82ef0294

...ecial '0' value indicates the time source is unreliable and we > + * need to use something else. The currently published specification > + * versions (up to 4.0b) contain a mistake and wrongly claim '-1' > + * instead of '0' as the special value, see commit c35b82ef0294. > + * - ReferenceTime = > + * ((RDTSC() * ReferenceTscScale) >> 64) + ReferenceTscOffset > + * - READ ReferenceTscSequence again. In case its value has changed > + * since our first reading we need to discard ReferenceTime and repeat > + * the whole sequence as...

...ecial '0' value indicates the time source is unreliable and we > + * need to use something else. The currently published specification > + * versions (up to 4.0b) contain a mistake and wrongly claim '-1' > + * instead of '0' as the special value, see commit c35b82ef0294. > + * - ReferenceTime = > + * ((RDTSC() * ReferenceTscScale) >> 64) + ReferenceTscOffset > + * - READ ReferenceTscSequence again. In case its value has changed > + * since our first reading we need to discard ReferenceTime and repeat > + * the whole sequence as...

...nvalid values - one for 'try again' and another for permanent failure in case it is needed. But this is not the case. So I keep my algorithm untouched. I can see two more reasons for us to keep it: 1) It is what we already have in kernel. 2) It is what Windows does (see the disassembly in c35b82ef0294. Original description: Hyper-V TSC page clocksource is suitable for vDSO, however, the protocol defined by the hypervisor is different from VCLOCK_PVCLOCK. Implemented the required support. Simple sysbench test shows the following results: Before: # time sysbench --test=memory --max-requests=500...

...nvalid values - one for 'try again' and another for permanent failure in case it is needed. But this is not the case. So I keep my algorithm untouched. I can see two more reasons for us to keep it: 1) It is what we already have in kernel. 2) It is what Windows does (see the disassembly in c35b82ef0294. Original description: Hyper-V TSC page clocksource is suitable for vDSO, however, the protocol defined by the hypervisor is different from VCLOCK_PVCLOCK. Implemented the required support. Simple sysbench test shows the following results: Before: # time sysbench --test=memory --max-requests=500...

On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote: > +#ifdef CONFIG_HYPERV_TSCPAGE > +static notrace u64 vread_hvclock(int *mode) > +{ > + const struct ms_hyperv_tsc_page *tsc_pg = > + (const struct ms_hyperv_tsc_page *)&hvclock_page; > + u64 sequence, scale, offset, current_tick, cur_tsc; > + > + while (1) { > + sequence = READ_ONCE(tsc_pg->tsc_sequence); > + if

On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote: > +#ifdef CONFIG_HYPERV_TSCPAGE > +static notrace u64 vread_hvclock(int *mode) > +{ > + const struct ms_hyperv_tsc_page *tsc_pg = > + (const struct ms_hyperv_tsc_page *)&hvclock_page; > + u64 sequence, scale, offset, current_tick, cur_tsc; > + > + while (1) { > + sequence = READ_ONCE(tsc_pg->tsc_sequence); > + if

...al '0' value indicates the time source is unreliable and we + * need to use something else. The currently published specification + * versions (up to 4.0b) contain a mistake and wrongly claim '-1' + * instead of '0' as the special value, see commit c35b82ef0294. + * - ReferenceTime = + * ((RDTSC() * ReferenceTscScale) >> 64) + ReferenceTscOffset + * - READ ReferenceTscSequence again. In case its value has changed + * since our first reading we need to discard ReferenceTime and repeat + * the whole sequen...

...nce + * A special '0' value indicates the time source is unreliable and we + * need to use something else. The currently published specification + * versions (up to 4.0b) contain a mistake and wrongly claim '-1' + * instead of '0' as the special value, see commit c35b82ef0294. + * - ReferenceTime = + * ((RDTSC() * ReferenceTscScale) >> 64) + ReferenceTscOffset + * - READ ReferenceTscSequence again. In case its value has changed + * since our first reading we need to discard ReferenceTime and repeat + * the whole sequence as the hypervisor was updat...

...V is the same we're safe with compile barriers only. > > 3. You need rdtsc_ordered(). Plain RDTSC is not ordered wrt other > memory access, and it can observably screw up as a result. Another interesting thing is if we look at how this is implemented in Windows (see linux.git commit c35b82ef0294) there are no barriers there even for rdtsc... > > Also, Linux tries to avoid volatile variables, so please use READ_ONCE(). > Will do both of the above in my next submission, thanks for the feedback! [snip] -- Vitaly

...V is the same we're safe with compile barriers only. > > 3. You need rdtsc_ordered(). Plain RDTSC is not ordered wrt other > memory access, and it can observably screw up as a result. Another interesting thing is if we look at how this is implemented in Windows (see linux.git commit c35b82ef0294) there are no barriers there even for rdtsc... > > Also, Linux tries to avoid volatile variables, so please use READ_ONCE(). > Will do both of the above in my next submission, thanks for the feedback! [snip] -- Vitaly

Thomas Gleixner <tglx at linutronix.de> writes: > On Tue, 14 Feb 2017, Vitaly Kuznetsov wrote: > >> Hi, >> >> while we're still waiting for a definitive ACK from Microsoft that the >> algorithm is good for SMP case (as we can't prevent the code in vdso from >> migrating between CPUs) I'd like to send v2 with some modifications to keep

Thomas Gleixner <tglx at linutronix.de> writes: > On Tue, 14 Feb 2017, Vitaly Kuznetsov wrote: > >> Hi, >> >> while we're still waiting for a definitive ACK from Microsoft that the >> algorithm is good for SMP case (as we can't prevent the code in vdso from >> migrating between CPUs) I'd like to send v2 with some modifications to keep

Hi, while we're still waiting for a definitive ACK from Microsoft that the algorithm is good for SMP case (as we can't prevent the code in vdso from migrating between CPUs) I'd like to send v2 with some modifications to keep the discussion going. Changes since v1: - Document the TSC page reading protocol [Thomas Gleixner]. - Separate the TSC page reading code from

Hi, while we're still waiting for a definitive ACK from Microsoft that the algorithm is good for SMP case (as we can't prevent the code in vdso from migrating between CPUs) I'd like to send v2 with some modifications to keep the discussion going. Changes since v1: - Document the TSC page reading protocol [Thomas Gleixner]. - Separate the TSC page reading code from