[Bug 221956] cam iosched: Schedule cam_iosched_ticker() quanta times per second

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221956

            Bug ID: 221956
           Summary: cam iosched: Schedule cam_iosched_ticker() quanta
                    times per second
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Keywords: patch
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: fk at fabiankeil.de
             Flags: mfc-stable11?

Created attachment 185947
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=185947&action=edit
cam iosched: Schedule cam_iosched_ticker() quanta times per second

The attached patch lets cam iosched schedule cam_iosched_ticker()
quanta times per second

According to callout_reset(9), "[w]hen ticks is used, the callout is
scheduled to execute after ticks/hz seconds".

It follows that to get quanta calls per second one should specify
a tick value of hz / isc->quanta.

Previously the value was additionally reduced by one.

As a result cam_iosched_ticker() was called more often than
requested by the user and expected by some of the limiters.

On a system with hz=1000 a quanta value of 200 resulted in 250
calls and a value of 100 in 111 calls.

Without the unexplained tick reduction the number of calls matches
the quanta value reasonably well.

Obtained from: ElectroBSD

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