![]() ![]() IRQL is primarily a serialization mechanism. It may be interrupted by a higher-IRQL task (which could in turn be interrupted by a yet higher-IRQL task, etc.), but when the higher-IRQL tasks complete, control returns to the task it interrupted. ![]() Once a kernel task is started at IRQL 2 or above, it runs to completion before anything else at the same IRQL will be started on the same processor. Each is a subset of what PerfMon shows as "% privileged time" - which should have been labeled "kernel mode time". Performance Monitor shows time spent at IRQL 2 as "% DPC time" and time at IRQL > 2 as "% interrupt time", regardless of whether the time was actually spent in a DPC routine or ISR or was a the result of raising IRQL from a lower value. Such threads start at IRQL 0, but if part of the code raises to IRQL 2 or above, does something, and then returns to its previous IRQL, the high-IRQL part of the code is one example of what I'm calling a "kernel task" here. But another example can be code in a kernel mode thread. As I (and others) use the term here, "kernel mode task" really covers "anything with a defined beginning and end that needs to be done in kernel mode at IRQL 2 or above." An interrupt service routine is a one example of a "kernel mode task" so is a DPC routine. processes and threads, and there is no relation to x86 "task gates" nor to anything shown in "Task Manager". Windows does not really call these things "kernel tasks", they are not managed objects as are e.g. ( I should also mention that the term "kernel task" here is not official. Unlike thread priorities, kernel tasks at the same IRQL are not time-sliced, and IRQLs aren't subject to automatic boost and decay. ( I use the word "importance" rather than "priority" because "priority" in Windows refers to thread priorities, and IRQLs are something different. This includes the "Logical Processors" created by hyperthreading. On a multiprocessor system each processor has its own IRQL. If a new task (such as an interrupt service routine) has a higher IRQL than the processor's current IRQL, then yes, it can interrupt the current task otherwise no. IRQL is a Windows-defined state of the processor - not of a process or thread - that indicates to Windows whether or not whatever that processor is doing can be interrupted by other tasks. It represents the "importance" of a kernel mode task relative to other kernel mode tasks. It is a number, ranging from 0 through 31 on Windows x86 systems and 0 through 15 on 圆4 systems. ![]() IRQL stands for "Interrupt Request Level". ![]()
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