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

-----Original Message----- > Sure. On my ppc64le machine, crash got 96hz after applying the above patch. The reason > is that kernel calculates the value of bfq_timeout as below: > > bfq_timeout = HZ / 8; > > The actual value of HZ is 100, so bfq_timeout = 100 / 8 = 12, but in crash, we calculate > the value of HZ: > > HZ = bfq_timeout * 8 = 12 * 8 = 96 > > It seems that this is not the result what we expected.

> >> 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) > > Sounds good. But unfortunately, the 'read_expire' is a static variable in kernel, we > can not get it directly by the symbol search. Maybe we should try to find a static > variable(kernel) in another ways. > > If it is possible, I would tend to use the 'write_expire' to calculate the value of HZ > in crash as below, that can avoid the above issues and get a correct result. > > HZ = write_expire / 5; > > /* > * source: block/mq-deadline.c > */ > static const int write_expire = 5 * HZ > > For example: > + if (symbol_exists("write_expire")) { ----> Here, it failed, maybe we can try to find the symbol in another way. > + uint write_expire; > + get_symbol_data("write_expire", sizeof(int), &write_expire); > + if (write_expire) { > + machdep->hz = write_expire / 5; > + if (CRASHDEBUG(2)) > + fprintf(fp, "write_expire exists: setting hz to %d\n", > + machdep->hz); > + } > + } else > >> that I have do not have a symbol for it. (some optimization?) >> > I can get the values of 'read_expire' and 'write_expire' in the latest rhel8 or later. > > crash> p read_expire > $1 = 50 > crash> p write_expire > $2 = 500 > > Thanks. > Linabo > How do you think about the following changes? It works for me. /* * source: net/ipv4/inetpeer.c * int inet_peer_minttl __read_mostly = 120 * HZ; /* TTL under high load: 120 sec */ */