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Revision 706a1ea65e6faaf853427a0e931f59d604dd45e3 authored by Linus Torvalds on 23 August 2018, 21:55:01 UTC, committed by Linus Torvalds on 23 August 2018, 21:55:01 UTC 
Merge branch 'tlb-fixes'
 
Merge fixes for missing TLB shootdowns.

This fixes a couple of cases that involved us possibly freeing page
table structures before the required TLB shootdown had been done.

There are a few cleanup patches to make the code easier to follow, and
to avoid some of the more problematic cases entirely when not necessary.

To make this easier for backports, it undoes the recent lazy TLB
patches, because the cleanups and fixes are more important, and Rik is
ok with re-doing them later when things have calmed down.

The missing TLB flush was only delayed, and the wrong ordering only
happened under memory pressure (and in theory under a couple of other
fairly theoretical situations), so this may have been all very unlikely
to have hit people in practice.

But getting the TLB shootdown wrong is _so_ hard to debug and see that I
consider this a crticial fix.

Many thanks to Jann Horn for having debugged this.

* tlb-fixes:
  x86/mm: Only use tlb_remove_table() for paravirt
  mm: mmu_notifier fix for tlb_end_vma
  mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE
  mm/tlb: Remove tlb_remove_table() non-concurrent condition
  mm: move tlb_table_flush to tlb_flush_mmu_free
  x86/mm/tlb: Revert the recent lazy TLB patches
2 parent s d40acad + 48a8b97
Raw File
cpu-load.txt 
========
CPU load
========

Linux exports various bits of information via ``/proc/stat`` and
``/proc/uptime`` that userland tools, such as top(1), use to calculate
the average time system spent in a particular state, for example::

    $ iostat
    Linux 2.6.18.3-exp (linmac)     02/20/2007

    avg-cpu:  %user   %nice %system %iowait  %steal   %idle
              10.01    0.00    2.92    5.44    0.00   81.63

    ...

Here the system thinks that over the default sampling period the
system spent 10.01% of the time doing work in user space, 2.92% in the
kernel, and was overall 81.63% of the time idle.

In most cases the ``/proc/stat``	 information reflects the reality quite
closely, however due to the nature of how/when the kernel collects
this data sometimes it can not be trusted at all.

So how is this information collected?  Whenever timer interrupt is
signalled the kernel looks what kind of task was running at this
moment and increments the counter that corresponds to this tasks
kind/state.  The problem with this is that the system could have
switched between various states multiple times between two timer
interrupts yet the counter is incremented only for the last state.


Example
-------

If we imagine the system with one task that periodically burns cycles
in the following manner::

     time line between two timer interrupts
    |--------------------------------------|
     ^                                    ^
     |_ something begins working          |
                                          |_ something goes to sleep
                                         (only to be awaken quite soon)

In the above situation the system will be 0% loaded according to the
``/proc/stat`` (since the timer interrupt will always happen when the
system is executing the idle handler), but in reality the load is
closer to 99%.

One can imagine many more situations where this behavior of the kernel
will lead to quite erratic information inside ``/proc/stat``::


	/* gcc -o hog smallhog.c */
	#include <time.h>
	#include <limits.h>
	#include <signal.h>
	#include <sys/time.h>
	#define HIST 10

	static volatile sig_atomic_t stop;

	static void sighandler (int signr)
	{
	(void) signr;
	stop = 1;
	}
	static unsigned long hog (unsigned long niters)
	{
	stop = 0;
	while (!stop && --niters);
	return niters;
	}
	int main (void)
	{
	int i;
	struct itimerval it = { .it_interval = { .tv_sec = 0, .tv_usec = 1 },
				.it_value = { .tv_sec = 0, .tv_usec = 1 } };
	sigset_t set;
	unsigned long v[HIST];
	double tmp = 0.0;
	unsigned long n;
	signal (SIGALRM, &sighandler);
	setitimer (ITIMER_REAL, &it, NULL);

	hog (ULONG_MAX);
	for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog (ULONG_MAX);
	for (i = 0; i < HIST; ++i) tmp += v[i];
	tmp /= HIST;
	n = tmp - (tmp / 3.0);

	sigemptyset (&set);
	sigaddset (&set, SIGALRM);

	for (;;) {
		hog (n);
		sigwait (&set, &i);
	}
	return 0;
	}


References
----------

- http://lkml.org/lkml/2007/2/12/6
- Documentation/filesystems/proc.txt (1.8)


Thanks
------

Con Kolivas, Pavel Machek
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