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    SubjectRe: WARNING: Adjusting tsc more then 11%
    On Mon, Mar 05, 2012 at 12:24:37PM -0800, John Stultz wrote:
    > > > Ok. Well, just to level set: the warning is informative, and points to
    > > > unexpected, but not necessarily unsafe behavior.
    > > >
    > > > In fact, the risk (where mult is adjusted to be large enough to cause an
    > > > overflow) we're warning about have been present 2.6.36 or even possibly
    > > > before. The change in 3.2 which added the warning also added a more
    > > > conservative mult calculation, so we're less likely to get overflow
    > > > prone large mult values.
    > >
    > > Is there a reason you decided to use a WARN_ONCE, which dumps a full stack
    > > trace, instead of just printk(KERN_ERR ?
    >
    > Well, the WARN_ONCE behavior is really nice, since just a printk would
    > end up possibly filling the logs, since you might get one every tick.

    We have printk_once too.

    > > > So it would be great to get further feedback from folks who are seeing
    > > > this warning, so we can really hammer this out, but I don't want the
    > > > warning spooking anyone into thinking things are terribly broken.
    > >
    > > Right... people see backtraces and start thinking "my kernel is broken."
    > >
    > > I'm certainly not meaning to pick on you for this. Lately it seems all
    > > the rage to throw WARN_ONs for all kinds of error paths and leave the user
    > > to figure out how screwed they are.
    >
    > Its a trade-off, since we really do want to know if our code has been
    > pushed outside of its expected boundaries (either by unexpected hadware
    > behavior or by expectations being raised, like long nohz idle times), so
    > we have to get folks attention somewhat. The type of error reporting
    > Dave's managed to collect here is really great.

    It is, yes. Do you know, aside from distro kernel maintainers, how many
    reports have you gotten from actual users directly?

    > But at the same time, I agree there has been a few cases where the code
    > is limited more narrowly then the reality of existing hardware, and we
    > end up with a constant stream of error messages that get waved off as
    > broken hardware.
    >
    > There we need to either fix the code or drop the warnings, but I think
    > it gets hard when we really want to know about "unexpected behavior,
    > except on some wide swath of hardware that always acts poorly", where
    > conditionalizing the warnings isn't easy.

    Oh my. Quirks in the timekeeping code would just give me nightmares ;).

    josh


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