https://github.com/torvalds/linux
Revision ecefc36b41ac0fe92d76273a23faf27b2da13411 authored by Linus Torvalds on 22 December 2011, 02:29:26 UTC, committed by Linus Torvalds on 22 December 2011, 02:29:26 UTC
* git://git.kernel.org/pub/scm/linux/kernel/git/davem/net: net: Add a flow_cache_flush_deferred function ipv4: reintroduce route cache garbage collector net: have ipconfig not wait if no dev is available sctp: Do not account for sizeof(struct sk_buff) in estimated rwnd asix: new device id davinci-cpdma: fix locking issue in cpdma_chan_stop sctp: fix incorrect overflow check on autoclose r8169: fix Config2 MSIEnable bit setting. llc: llc_cmsg_rcv was getting called after sk_eat_skb. net: bpf_jit: fix an off-one bug in x86_64 cond jump target iwlwifi: update SCD BC table for all SCD queues Revert "Bluetooth: Revert: Fix L2CAP connection establishment" Bluetooth: Clear RFCOMM session timer when disconnecting last channel Bluetooth: Prevent uninitialized data access in L2CAP configuration iwlwifi: allow to switch to HT40 if not associated iwlwifi: tx_sync only on PAN context mwifiex: avoid double list_del in command cancel path ath9k: fix max phy rate at rate control init nfc: signedness bug in __nci_request() iwlwifi: do not set the sequence control bit is not needed
Tip revision: ecefc36b41ac0fe92d76273a23faf27b2da13411 authored by Linus Torvalds on 22 December 2011, 02:29:26 UTC
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
Tip revision: ecefc36
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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