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Re: [PATCH v2 4/4] MIPS: perf: Add support for 64-bit perf counters.

To: Deng-Cheng Zhu <dengcheng.zhu@gmail.com>, ralf@linux-mips.org
Subject: Re: [PATCH v2 4/4] MIPS: perf: Add support for 64-bit perf counters.
From: David Daney <ddaney@caviumnetworks.com>
Date: Thu, 17 Feb 2011 09:26:17 -0800
Cc: linux-mips@linux-mips.org, Peter Zijlstra <a.p.zijlstra@chello.nl>, Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@elte.hu>, Arnaldo Carvalho de Melo <acme@redhat.com>
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On 02/17/2011 02:46 AM, Deng-Cheng Zhu wrote:
Hi, David


The reason of the perf-record failure on 32bit platforms is that the 32bit
counter read function mipsxx_pmu_read_counter() returns wrong 64bit values.
For example, the counter value 0x12345678 will be returned as
0xffffffff12345678. So in mipspmu_event_update(), the delta will be wrong.
So here's a possible fix for your reference:


Thanks for the excellent detective work. I will generate a fixed patch set. I think we are getting close to something that will work here.

David Daney



--- a/arch/mips/kernel/perf_event_mipsxx.c
+++ b/arch/mips/kernel/perf_event_mipsxx.c
@@ -184,19 +184,21 @@ static unsigned int
mipsxx_pmu_swizzle_perf_idx(unsigned int idx)
         return idx;
  }

+#define U32_MASK 0xffffffff
+
  static u64 mipsxx_pmu_read_counter(unsigned int idx)
  {
         idx = mipsxx_pmu_swizzle_perf_idx(idx);

         switch (idx) {
         case 0:
-               return read_c0_perfcntr0();
+               return read_c0_perfcntr0()&  U32_MASK;
         case 1:
-               return read_c0_perfcntr1();
+               return read_c0_perfcntr1()&  U32_MASK;
         case 2:
-               return read_c0_perfcntr2();
+               return read_c0_perfcntr2()&  U32_MASK;
         case 3:
-               return read_c0_perfcntr3();
+               return read_c0_perfcntr3()&  U32_MASK;
         default:
                 WARN_ONCE(1, "Invalid performance counter number (%d)\n", idx);
                 return 0;

In addition, since you removed the use of cpuc->msbs, some code relative to
this logic can be removed:

@@ -370,7 +372,6 @@ static int mipspmu_event_set_period(struct
perf_event *event,
         u64 left = local64_read(&hwc->period_left);
         u64 period = hwc->sample_period;
         int ret = 0;
-       unsigned long flags;

         if (unlikely((left + period)&  (1ULL<<  63))) {
                 /* left underflowed by more than period. */
@@ -393,9 +394,7 @@ static int mipspmu_event_set_period(struct
perf_event *event,

         local64_set(&hwc->prev_count, mipspmu.overflow - left);

-       local_irq_save(flags);
         mipspmu.write_counter(idx, mipspmu.overflow - left);
-       local_irq_restore(flags);

         perf_event_update_userpage(event);

@@ -406,16 +405,12 @@ static void mipspmu_event_update(struct perf_event *event,
                                  struct hw_perf_event *hwc,
                                  int idx)
  {
-       unsigned long flags;
         u64 prev_raw_count, new_raw_count;
         u64 delta;

  again:
         prev_raw_count = local64_read(&hwc->prev_count);
-       local_irq_save(flags);
-       /* Make the counter value be a "real" one. */
         new_raw_count = mipspmu.read_counter(idx);
-       local_irq_restore(flags);

         if (local64_cmpxchg(&hwc->prev_count, prev_raw_count,
                                 new_raw_count) != prev_raw_count)

And here's a general comment: You are putting the majority of the
implementation in perf_event_mipsxx.c. This will require other CPUs like
Loongson2 to replicate quite a lot code in their corresponding files. I
personally think the original "skeleton + #include perf_event_$cpu.c" is a
better choice. I understand you prefer not using code like
"#if defined(CONFIG_CPU_MIPS32)" on the top of perf_event_$cpu.c, but that
is what other architectures (X86/ARM etc) are doing.


Deng-Cheng


2011/1/28 Deng-Cheng Zhu<dengcheng.zhu@gmail.com>:
OK. I'll try to use tracing when needed.

The hardware I was using for the test was Malta-R with 34K bitfile
programed into the FPGA. The CPU frequency is 50MHz.


Deng-Cheng


2011/1/28 David Daney<ddaney@caviumnetworks.com>:
On 01/26/2011 10:24 PM, Deng-Cheng Zhu wrote:

Using your attached patch, I experimented -c and -F by 'ls /'. The numbers
I used are 10, 1000 and 100000 for both -c and -F.

The number of samples I got was 24 all the way. That means the event
period
to sample and the profiling frequency do not affect the results on MIPS32
platform. While working on the old code, the system had the following
results:

-c 10: The system seems busy dealing with interrupts. And the following
log
        was printed out:
        ================================================
        hda: ide_dma_sff_timer_expiry: DMA status (0x24)
        hda: DMA interrupt recovery
        hda: lost interrupt
        ================================================
        This does need to be fixed later on.
-c 1000: ~11085 samples
-c 100000: ~48 samples ('perf report' still showed some data.)
-F 10: ~118 samples
-F 1000: ~352 samples
-F 100000: ~379 samples

I'll try to take time to look into the patch to see if anything can be
changed.


I have found it useful to enable tracing, and then placing trace_printk() in
mipspmu_event_set_period() to look at the values of:

sample_period, period_left that are being used.

Also you could use a trace_printk() in mipsxx_pmu_write_counter() to check
the value being written to the register.

What hardware are you using to test this?  I wonder if there is a board with
a 32-bit CPU that I could get access to.

David Daney



Deng-Cheng


2011/1/26 David Daney<ddaney@caviumnetworks.com>:

On 01/24/2011 07:42 PM, Deng-Cheng Zhu wrote:

Hi, David


This version does fix the problem with 'perf stat'. However, when
working
with 'perf record', the following happened:

-sh-4.0# perf record -f -e cycles -e instructions -e branches \

-e branch-misses -e r12 find / -name "*sys*">/dev/null

[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.001 MB perf.data (~53 samples) ]


I get the same thing.  What happens if you supply either '-c xxx' or '-f
xxx'?

I get:octeon:~/linux/tools/perf# ./perf record -e cycles /bin/ls -l /
total 100
drwxr-xr-x   2 root root  4096 2010-11-12 11:39 bin
[...]
drwxr-xr-x  13 root root  4096 2007-05-25 12:28 var
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.002 MB perf.data (~82 samples) ]

Almost no samples as you got.

But if I do:

octeon:~/linux/tools/perf# ./perf record -F 100000 -e cycles /bin/ls -l /
total 100
drwxr-xr-x   2 root root  4096 2010-11-12 11:39 bin
[...]
drwxr-xr-x  13 root root  4096 2007-05-25 12:28 var
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.404 MB perf.data (~17653 samples) ]

Look many more samples!

The question is, what is it supposed to do?

If you can get a reasonable number of samples out if you supply -c or
-F, then I would argue that it is working and the default settings for
-F are not a good fit for your test case.

I have slightly changed the patch.  You could try the attached version
instead and tell me the results.


David Daney










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