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Classic SRCU in effect expedites the first synchronize_srcu() when SRCU is idle, and Mike Galbraith demonstrated that some use cases do in fact rely on this behavior. In particular, Mike showed that Steven Rostedt's hotplug stress script takes 55 seconds with Classic SRCU and more than 16 -minutes- when running Tree SRCU. Assuming that each Tree SRCU's call to synchronize_srcu() takes four milliseconds, this implies that Steven's test invokes synchronize_srcu() in isolation, but more than once per 200 microseconds. Mike used ftrace to demonstrate that the time between successive calls to synchronize_srcu() ranged from 118 to 342 microseconds, with one outlier at 80 milliseconds. This data clearly indicates that Tree SRCU needs to expedite the first invocation of synchronize_srcu() during an SRCU idle period. This commit therefor introduces a srcu_might_be_idle() function that probabilistically checks whether or not SRCU is idle. This function is used by synchronize_rcu() as an additional criterion in deciding whether or not to expedite. (Hat trick to Peter Zijlstra for his earlier suggestion that this might in fact be a problem. Which for all I know might have motivated Mike to look into it.) Reported-by: Mike Galbraith <efault@gmx.de> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Mike Galbraith <efault@gmx.de> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.