Re: [PATCH 1/2] x86/tsc: Add new BPF helper call bpf_rdtsc

[Tero Kristo <tero.kristo@linux.intel.com>]

On 07/07/2023 08:41, John Fastabend wrote:
> Alexei Starovoitov wrote:
>> On Thu, Jul 6, 2023 at 4:59 AM Tero Kristo <tero.kristo@linux.intel.com> wrote:
>>>
>>> On 06/07/2023 08:16, John Fastabend wrote:
>>>> Alexei Starovoitov wrote:
>>>>> On Mon, Jul 3, 2023 at 3:58 AM Tero Kristo <tero.kristo@linux.intel.com> wrote:
>>>>>> Currently the raw TSC counter can be read within kernel via rdtsc_ordered()
>>>>>> and friends, and additionally even userspace has access to it via the
>>>>>> RDTSC assembly instruction. BPF programs on the other hand don't have
>>>>>> direct access to the TSC counter, but alternatively must go through the
>>>>>> performance subsystem (bpf_perf_event_read), which only provides relative
>>>>>> value compared to the start point of the program, and is also much slower
>>>>>> than the direct read. Add a new BPF helper definition for bpf_rdtsc() which
>>>>>> can be used for any accurate profiling needs.
>>>>>>
>>>>>> A use-case for the new API is for example wakeup latency tracing via
>>>>>> eBPF on Intel architecture, where it is extremely beneficial to be able
>>>>>> to get raw TSC timestamps and compare these directly to the value
>>>>>> programmed to the MSR_IA32_TSC_DEADLINE register. This way a direct
>>>>>> latency value from the hardware interrupt to the execution of the
>>>>>> interrupt handler can be calculated. Having the functionality within
>>>>>> eBPF also has added benefits of allowing to filter any other relevant
>>>>>> data like C-state residency values, and also to drop any irrelevant
>>>>>> data points directly in the kernel context, without passing all the
>>>>>> data to userspace for post-processing.
>>>>>>
>>>>>> Signed-off-by: Tero Kristo <tero.kristo@linux.intel.com>
>>>>>> ---
>>>>>>    arch/x86/include/asm/msr.h |  1 +
>>>>>>    arch/x86/kernel/tsc.c      | 23 +++++++++++++++++++++++
>>>>>>    2 files changed, 24 insertions(+)
>>>>>>
>>>>>> diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
>>>>>> index 65ec1965cd28..3dde673cb563 100644
>>>>>> --- a/arch/x86/include/asm/msr.h
>>>>>> +++ b/arch/x86/include/asm/msr.h
>>>>>> @@ -309,6 +309,7 @@ struct msr *msrs_alloc(void);
>>>>>>    void msrs_free(struct msr *msrs);
>>>>>>    int msr_set_bit(u32 msr, u8 bit);
>>>>>>    int msr_clear_bit(u32 msr, u8 bit);
>>>>>> +u64 bpf_rdtsc(void);
>>>>>>
>>>>>>    #ifdef CONFIG_SMP
>>>>>>    int rdmsr_on_cpu(unsigned int cpu, u32 msr_no, u32 *l, u32 *h);
>>>>>> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
>>>>>> index 344698852146..ded857abef81 100644
>>>>>> --- a/arch/x86/kernel/tsc.c
>>>>>> +++ b/arch/x86/kernel/tsc.c
>>>>>> @@ -15,6 +15,8 @@
>>>>>>    #include <linux/timex.h>
>>>>>>    #include <linux/static_key.h>
>>>>>>    #include <linux/static_call.h>
>>>>>> +#include <linux/btf.h>
>>>>>> +#include <linux/btf_ids.h>
>>>>>>
>>>>>>    #include <asm/hpet.h>
>>>>>>    #include <asm/timer.h>
>>>>>> @@ -29,6 +31,7 @@
>>>>>>    #include <asm/intel-family.h>
>>>>>>    #include <asm/i8259.h>
>>>>>>    #include <asm/uv/uv.h>
>>>>>> +#include <asm/tlbflush.h>
>>>>>>
>>>>>>    unsigned int __read_mostly cpu_khz;    /* TSC clocks / usec, not used here */
>>>>>>    EXPORT_SYMBOL(cpu_khz);
>>>>>> @@ -1551,6 +1554,24 @@ void __init tsc_early_init(void)
>>>>>>           tsc_enable_sched_clock();
>>>>>>    }
>>>>>>
>>>>>> +u64 bpf_rdtsc(void)
>>>>>> +{
>>>>>> +       /* Check if Time Stamp is enabled only in ring 0 */
>>>>>> +       if (cr4_read_shadow() & X86_CR4_TSD)
>>>>>> +               return 0;
>>>>> Why check this? It's always enabled in the kernel, no?
>>> It is always enabled, but there are certain syscalls that can be used to
>>> disable the TSC access for oneself. prctl(PR_SET_TSC, ...) and
>>> seccomp(SET_MODE_STRICT,...). Not having the check in place would in
>>> theory allow a restricted BPF program to circumvent this (if there ever
>>> was such a thing.) But yes, I do agree this part is a bit debatable
>>> whether it should be there at all.
>> What do you mean 'circumvent' ?
>> It's a tracing bpf prog running in the kernel loaded by root
>> and reading tsc for the purpose of the kernel.
>> There is no unprivileged access to tsc here.
This was based on some discussions with the security team at Intel, I 
don't pretend to know anything about security myself. But I can drop the 
check. It is probably not needed because of the fact that it is already 
possible to read the TSC counter with the approach I mention in the 
cover letter; via perf and bpf_core_read().
>>
>>>>>> +
>>>>>> +       return rdtsc_ordered();
>>>>> Why _ordered? Why not just rdtsc ?
>>>>> Especially since you want to trace latency. Extra lfence will ruin
>>>>> the measurements.
>>>>>
>>>> If we used it as a fast way to order events on multiple CPUs I
>>>> guess we need the lfence? We use ktime_get_ns() now for things
>>>> like this when we just need an order counter. We have also
>>>> observed time going backwards with this and have heuristics
>>>> to correct it but its rare.
>>> Yeah, I think it is better to induce some extra latency instead of
>>> having some weird ordering issues with the timestamps.
>> lfence is not 'some extra latency'.
>> I suspect rdtsc_ordered() will be slower than bpf_ktime_get_ns().
>> What's the point of using it then?
> I would only use it if its faster then bpf_ktime_get_ns() and
> have already figured out how to handle rare unordered events
> so I think its OK to relax somewhat strict ordering.

I believe that on x86-arch using bpf_ktime_get_ns() also ends up calling 
rdtsc_odered() under the hood.

I just did some measurements on an Intel(R) Xeon(R) Platinum 8360Y CPU @ 
2.40GHz, with a simple BPF code:

         t1 = bpf_ktime_get_ns();

         for (i = 0; i < NUM_CYC; i++) {
                 bpf_rdtsc(); // or bpf_ktime_get_ns() here
         }

         t2 = bpf_ktime_get_ns();

The results I got with the CPU locked at 2.4GHz (average execution times 
per a call within the loop, this with some 10M executions):

bpf_rdtsc() ordered : 45ns

bpf_rdtsc() un-ordered : 23ns

bpf_ktime_get_ns() : 49ns

Locking the CPU at 800MHz the results are:

bpf_rdtsc() ordered : 55ns

bpf_rdtsc() un-ordered : 33ns

bpf_ktime_get_ns() : 71ns

The bpf_rdtsc() in these results contains some extra latency caused by 
conditional execution, I added a flag to the call to select whether it 
should use _ordered() or not, and it also still contains the CR4_TSD 
check in place.

-Tero

>
>>> Also, things like the ftrace also use rdtsc_ordered() as its underlying
>>> clock, if you use x86-tsc as the trace clock (see
>>> arch/x86/kernel/trace_clock.c.)
>>>
>>> -Tero
>>>
>