Re: [PATCH v2 05/17] clocksource/hyperv: use MSR-based access if running as root

[Daniel Lezcano]

On 12/11/2020 12:24, Wei Liu wrote:
> On Thu, Nov 12, 2020 at 10:56:17AM +0100, Daniel Lezcano wrote:
>> On 05/11/2020 17:58, Wei Liu wrote:
>>> Signed-off-by: Wei Liu 

Acked-by: Daniel Lezcano 

>>> ---
>>
>> I would like to apply this patch but the changelog is too short (one line).
>>
>> Please add a small paragraph (no need to resend just answer here, I will
>> amend the log myself.
> 
> Please don't apply this to your tree. It is dependent on a previous
> patch. I expect this series to go through the hyperv tree.
> 
> I will add in this small paragraph in a later version:
> 
> When Linux runs as the root partition, the setup required for TSC page
> is different. Luckily Linux also has access to the MSR based
> clocksource. We can just disable the TSC page clocksource if Linux is
> the root partition.
> 
> If you're happy with the description, I would love to have an ack from
> you. I will funnel the patch via the hyperv tree.
> 
> Wei.
> 


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Re: [PATCH v2 05/17] clocksource/hyperv: use MSR-based access if running as root

[Daniel Lezcano]

On 05/11/2020 17:58, Wei Liu wrote:
> Signed-off-by: Wei Liu 
> ---

I would like to apply this patch but the changelog is too short (one line).

Please add a small paragraph (no need to resend just answer here, I will
amend the log myself.

>  drivers/clocksource/hyperv_timer.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/clocksource/hyperv_timer.c 
> b/drivers/clocksource/hyperv_timer.c
> index ba04cb381cd3..269a691bd2c4 100644
> --- a/drivers/clocksource/hyperv_timer.c
> +++ b/drivers/clocksource/hyperv_timer.c
> @@ -426,6 +426,9 @@ static bool __init hv_init_tsc_clocksource(void)
>   if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
>   return false;
>  
> + if (hv_root_partition)
> + return false;
> +
>   hv_read_reference_counter = read_hv_clock_tsc;
>   phys_addr = virt_to_phys(hv_get_tsc_page());
>  
> 


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Re: [PATCH RFC v3 3/6] sched/idle: Add a generic poll before enter real idle path

[Daniel Lezcano]

On 20/11/2017 08:05, Quan Xu wrote:

[ ... ]

 But the irq_timings stuff is heading into the same direction, with a
 more
 complex prediction logic which should tell you pretty good how long
 that
 idle period is going to be and in case of an interrupt heavy workload
 this
 would skip the extra work of stopping and restarting the tick and
 provide a
 very good input into a polling decision.
>>>
>>> interesting. I have tested with IRQ_TIMINGS related code, which seems
>>> not working so far.
>> I don't know how you tested it, can you elaborate what you meant by
>> "seems not working so far" ?
> 
> Daniel, I tried to enable IRQ_TIMINGS* manually. used
> irq_timings_next_event()
> to return estimation of the earliest interrupt. However I got a constant.

The irq timings gives you an indication of the next interrupt deadline.

This information is a piece of the puzzle, you need to combine it with
the next timer expiration, and the next scheduling event. Then take the
earliest event in a timeline basis.

Using the trivial scheme above will work well with workload like videos
or mp3 but will fail as soon as the interrupts are not coming in a
regular basis and this is where the pattern recognition algorithm must act.

>> There are still some work to do to be more efficient. The prediction
>> based on the irq timings is all right if the interrupts have a simple
>> periodicity. But as soon as there is a pattern, the current code can't
>> handle it properly and does bad predictions.
>>
>> I'm working on a self-learning pattern detection which is too heavy for
>> the kernel, and with it we should be able to detect properly the
>> patterns and re-ajust the period if it changes. I'm in the process of
>> making it suitable for kernel code (both math and perf).
>>
>> One improvement which can be done right now and which can help you is
>> the interrupts rate on the CPU. It is possible to compute it and that
>> will give an accurate information for the polling decision.
>>
>>
> As tglx said, talk to each other / work together to make it usable for
> all use cases.
> could you share how to enable it to get the interrupts rate on the CPU?
> I can try it
> in cloud scenario. of course, I'd like to work with you to improve it.

Sure, I will be glad if we can collaborate. I have some draft code but
before sharing it I would like we define what is the rate and what kind
of information we expect to infer from it. From my point of view it is a
value indicating the interrupt period per CPU, a short value indicates a
high number of interrupts on the CPU.

This value must decay with the time, the question here is what decay
function we apply to the rate from the last timestamp ?




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Re: [PATCH RFC v3 3/6] sched/idle: Add a generic poll before enter real idle path

[Daniel Lezcano]

On 16/11/2017 10:12, Quan Xu wrote:
> 
> 
> On 2017-11-16 06:03, Thomas Gleixner wrote:
>> On Wed, 15 Nov 2017, Peter Zijlstra wrote:
>>
>>> On Mon, Nov 13, 2017 at 06:06:02PM +0800, Quan Xu wrote:
 From: Yang Zhang 

 Implement a generic idle poll which resembles the functionality
 found in arch/. Provide weak arch_cpu_idle_poll function which
 can be overridden by the architecture code if needed.
>>> No, we want less of those magic hooks, not more.
>>>
 Interrupts arrive which may not cause a reschedule in idle loops.
 In KVM guest, this costs several VM-exit/VM-entry cycles, VM-entry
 for interrupts and VM-exit immediately. Also this becomes more
 expensive than bare metal. Add a generic idle poll before enter
 real idle path. When a reschedule event is pending, we can bypass
 the real idle path.
>>> Why not do a HV specific idle driver?
>> If I understand the problem correctly then he wants to avoid the heavy
>> lifting in tick_nohz_idle_enter() in the first place, but there is
>> already
>> an interesting quirk there which makes it exit early.  See commit
>> 3c5d92a0cfb5 ("nohz: Introduce arch_needs_cpu"). The reason for this
>> commit
>> looks similar. But lets not proliferate that. I'd rather see that go
>> away.
> 
> agreed.
> 
> Even we can get more benifit than commit 3c5d92a0cfb5 ("nohz: Introduce
> arch_needs_cpu")
> in kvm guest. I won't proliferate that..
> 
>> But the irq_timings stuff is heading into the same direction, with a more
>> complex prediction logic which should tell you pretty good how long that
>> idle period is going to be and in case of an interrupt heavy workload
>> this
>> would skip the extra work of stopping and restarting the tick and
>> provide a
>> very good input into a polling decision.
> 
> 
> interesting. I have tested with IRQ_TIMINGS related code, which seems
> not working so far.

I don't know how you tested it, can you elaborate what you meant by
"seems not working so far" ?

There are still some work to do to be more efficient. The prediction
based on the irq timings is all right if the interrupts have a simple
periodicity. But as soon as there is a pattern, the current code can't
handle it properly and does bad predictions.

I'm working on a self-learning pattern detection which is too heavy for
the kernel, and with it we should be able to detect properly the
patterns and re-ajust the period if it changes. I'm in the process of
making it suitable for kernel code (both math and perf).

One improvement which can be done right now and which can help you is
the interrupts rate on the CPU. It is possible to compute it and that
will give an accurate information for the polling decision.



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