Re: [Crash-utility] [PATCH] Fix machdep->HZ calculation for kernel versions > 2.6.0
-----Original Message-----
在 2021年01月12日 16:24, HAGIO KAZUHITO(萩尾 一仁) 写道:
> Hi Bhupesh,
>
> -----Original Message-----
>> We have hard-coded the HZ value for some ARCHs to either 1000 or 100
>> (mainly for kernel versions > 2.6.0), which causes 'help -m' to show
>> an incorrect hz value for various architectures.
>
> Good catch. but seems crash uses (cfq_slice_async * 25) for machdep->hz
> if it exists (please see task_init()), RHEL7 has it, but RHEL8 does not.
> What do you see on RHEL8 for x86_64 with your patch?
>
The symbol 'cfq_slice_async' has been removed from upstream kernel:
f382fb0bcef4 ("block: remove legacy IO schedulers")
And RHEL8 also removed it.
> We should search for an alternate way like the current one first.
>
Currently, there are several ways to get the value of HZ as below:
[1] calculate hz via the symbol 'cfq_slice_async'
But this symbol has been removed from upstream kernel
According to [0] below, the 'cfq_slice_async' cannot be used for the HZ
calculation on 4.8 and later kernels. I've not found a perfect alternate,
but how about using 'bfq_timeout' for 4.12 and later including RHEL8?
const int bfq_timeout = HZ / 8;
RELEASE: 4.18.0-80.el8.x86_64
crash> p bfq_timeout
bfq_timeout = $1 = 125
This value has not been changed since its introduction (aee69d78dec0).
Recent kernels configured with CONFIG_IOSCHED_BFQ=y can be covered with this?
[0] https://listman.redhat.com/archives/crash-utility/2021-April/msg00026.html
Thanks,
Kazu
[2] hardcode hz with the value 1000 (if kernel version > 2.6.0)
[3] get the hz value from vmcore, but that relies on kernel config
such as CONFIG_IKCONFIG, etc.
[4] Use sysconf(_SC_CLK_TCK) on some arches, not all arches.
See the micro definition of HZ in the defs.h
There seems to be no perfect solution. Any ideas?
Thanks.
Lianbo
> Thanks,
> Kazu
>
>>
>> I tested this on ppc64le and x86_64 and the hz value reported is 1000,
>> whereas the kernel CONFIG_HZ_100 is set to Y. See some logs below:
>>
>> crash> help -m
>> flags: 124000f5
>>
(KSYMS_START|MACHDEP_BT_TEXT|VM_4_LEVEL|VMEMMAP|VMEMMAP_AWARE|PHYS_ENTRY_L4|SWAP_ENTRY_L4|RADIX_MMU|OP
>> AL_FW)
>> kvbase: c000000000000000
>> identity_map_base: c000000000000000
>> pagesize: 65536
>> pageshift: 16
>> pagemask: ffffffffffff0000
>> pageoffset: ffff
>> stacksize: 16384
>> hz: 1000
>> mhz: 2800
>>
>> [host@rhel7]$ grep CONFIG_HZ_100= redhat/configs/kernel-3.10.0-ppc64le.config
>> CONFIG_HZ_100=y
>>
>> Fix the same by using the sysconf(_SC_CLK_TCK) value instead of the
>> hardcoded HZ values depending on kernel versions.
>>
>