Re: [Crash-utility] [PATCH] Fix machdep->HZ calculation for kernel versions > 2.6.0

-----Original Message----- > -----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? e.g. like this: --- a/task.c +++ b/task.c @@ -417,7 +417,16 @@ task_init(void) STRUCT_SIZE_INIT(cputime_t, "cputime_t"); - if (symbol_exists("cfq_slice_async")) { + if (symbol_exists("bfq_timeout")) { + uint bfq_timeout; + get_symbol_data("bfq_timeout", sizeof(int), &bfq_timeout); + if (bfq_timeout) { + machdep->hz = bfq_timeout * 8; + if (CRASHDEBUG(2)) + fprintf(fp, "bfq_timeout exists: setting hz to %d\n", + machdep->hz); + } + } else if (symbol_exists("cfq_slice_async")) { uint cfq_slice_async; get_symbol_data("cfq_slice_async", sizeof(int), Lianbo, could you try this on ppc64le if it looks good?

btw, I thought 'read_expire' was better than the 'bfq_timeout' because it was introduced at 2.6.16 and has been unchanged, but most of kernels(vmlinux)

that I have do not have a symbol for it. (some optimization?)

static const int read_expire = HZ / 2; /* max time before a read is submitted. */ RELEASE: 4.18.0-80.el8.x86_64 crash> p read_expire No symbol "read_expire" in current context. p: gdb request failed: p read_expire Thanks, Kazu > > 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. >>>> >>> > > > -- > Crash-utility mailing list > Crash-utility(a)redhat.com > https://listman.redhat.com/mailman/listinfo/crash-utility