cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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cpu-load.rst (3103B)


      1========
      2CPU load
      3========
      4
      5Linux exports various bits of information via ``/proc/stat`` and
      6``/proc/uptime`` that userland tools, such as top(1), use to calculate
      7the average time system spent in a particular state, for example::
      8
      9    $ iostat
     10    Linux 2.6.18.3-exp (linmac)     02/20/2007
     11
     12    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
     13              10.01    0.00    2.92    5.44    0.00   81.63
     14
     15    ...
     16
     17Here the system thinks that over the default sampling period the
     18system spent 10.01% of the time doing work in user space, 2.92% in the
     19kernel, and was overall 81.63% of the time idle.
     20
     21In most cases the ``/proc/stat``	 information reflects the reality quite
     22closely, however due to the nature of how/when the kernel collects
     23this data sometimes it can not be trusted at all.
     24
     25So how is this information collected?  Whenever timer interrupt is
     26signalled the kernel looks what kind of task was running at this
     27moment and increments the counter that corresponds to this tasks
     28kind/state.  The problem with this is that the system could have
     29switched between various states multiple times between two timer
     30interrupts yet the counter is incremented only for the last state.
     31
     32
     33Example
     34-------
     35
     36If we imagine the system with one task that periodically burns cycles
     37in the following manner::
     38
     39     time line between two timer interrupts
     40    |--------------------------------------|
     41     ^                                    ^
     42     |_ something begins working          |
     43                                          |_ something goes to sleep
     44                                         (only to be awaken quite soon)
     45
     46In the above situation the system will be 0% loaded according to the
     47``/proc/stat`` (since the timer interrupt will always happen when the
     48system is executing the idle handler), but in reality the load is
     49closer to 99%.
     50
     51One can imagine many more situations where this behavior of the kernel
     52will lead to quite erratic information inside ``/proc/stat``::
     53
     54
     55	/* gcc -o hog smallhog.c */
     56	#include <time.h>
     57	#include <limits.h>
     58	#include <signal.h>
     59	#include <sys/time.h>
     60	#define HIST 10
     61
     62	static volatile sig_atomic_t stop;
     63
     64	static void sighandler(int signr)
     65	{
     66		(void) signr;
     67		stop = 1;
     68	}
     69
     70	static unsigned long hog (unsigned long niters)
     71	{
     72		stop = 0;
     73		while (!stop && --niters);
     74		return niters;
     75	}
     76
     77	int main (void)
     78	{
     79		int i;
     80		struct itimerval it = {
     81			.it_interval = { .tv_sec = 0, .tv_usec = 1 },
     82			.it_value    = { .tv_sec = 0, .tv_usec = 1 } };
     83		sigset_t set;
     84		unsigned long v[HIST];
     85		double tmp = 0.0;
     86		unsigned long n;
     87		signal(SIGALRM, &sighandler);
     88		setitimer(ITIMER_REAL, &it, NULL);
     89
     90		hog (ULONG_MAX);
     91		for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog(ULONG_MAX);
     92		for (i = 0; i < HIST; ++i) tmp += v[i];
     93		tmp /= HIST;
     94		n = tmp - (tmp / 3.0);
     95
     96		sigemptyset(&set);
     97		sigaddset(&set, SIGALRM);
     98
     99		for (;;) {
    100			hog(n);
    101			sigwait(&set, &i);
    102		}
    103		return 0;
    104	}
    105
    106
    107References
    108----------
    109
    110- https://lore.kernel.org/r/loom.20070212T063225-663@post.gmane.org
    111- Documentation/filesystems/proc.rst (1.8)
    112
    113
    114Thanks
    115------
    116
    117Con Kolivas, Pavel Machek