Windows® Internals, Sixth Edition, Part 1

A second possibility is that the generation of the per-CPU quota entry contained in the current process’ CPU quota block (owned by the session) does not manage the generation of the current per-CPU DFSS data structure. This generation mismatch suggests that a new interval has been reached and no cycle limits have yet been set, so PspReplenishCycleCredit is called to do the work. This reads the per-CPU weight and the total weight that were captured earlier in PspCalculateCpuQuotaBlockCycleCredits, and it uses them to set the base cycle allowance for the current per-CPU data inside the process’ CPU quota block. To do this, it uses a simple formula: the process receives the equivalent of its cycle credit (150 ms) divided by the total weight of all sessions on the system. Then the amount of cycles it will be permitted to run for (CyclesRemaining) is set to the base cycle allowance multiplied by the weight of this particular session. In other words, the process runs for a fairly-divided chunk of time based on the number of other sessions on the system, calculated as a percentage based on its relative weight compared to the overall system weight. When the computation is completed, the generation is set to match.

In all other cases, PsChargeProcessCpuCycles merely subtracts the amount of cycles from CyclesRemaining and then calls PsCheckThreadCpuQuota to see whether these cycles have been exhausted (reaching zero). Note that this function can sometimes also be called directly from the context switch code when control is about to pass to a thread that has CPU throttling enabled.

PsCheckThreadCpuQuota recovers the CPU quota block for this process (that is, for the session), and then further extracts the precise per-CPU information out of it. Once again, it checks whether the generation does not match, which would indicate this is the first charge for this 150-ms credit cycle, and then it calls PspReplenishCycleCredit. Next, it checks whether the CPU quota block for the process indicates there are no more cycles remaining. If cycles still remain, the function returns; otherwise, it prepares to suspend the thread’s execution.

Before stopping execution, the function extracts the per-CPU DPC, making sure that it (or the associated per-thread APC) is not already running. If this operation is happening due to the context-switch scenario brought up earlier, the per-thread APC is queued, which will preempt the thread’s execution as soon as the context switch completes. Otherwise, if this is occurring as result of cycle charging (which happens at DISPATCH_LEVEL or higher), the per-CPU DPC is queued instead, which will later queue the per-thread APC. (This forces a near-immediate response to the CPU quota restriction.) In case further cycle accumulation has occurred past the 150-ms cycle credit, the DPC also calls PspStartNewFairShareInterval, which was explained earlier.

CPU Throttling and Quota Enforcement

So far, you’ve seen how DFSS initializes, how CPU quota blocks are created for each session (and then associated with member processes), and how threads running with the CPU throttling bit (implying they are part of processes that are members of a session with DFSS enabled) will consume cycles out of their total weight-relative allowance, resetting every 150 ms. You also saw how, eventually, an APC is queued in all cases where a thread has exhausted its allowed cycles. You’ll now see how the APC enforces the CPU quota restriction.

The APC first enters an infinite loop, creating a stack-allocated Quota Wait Block that contains the current thread being restricted, as well as a resume event. It is this event that ultimately allows the thread to continue its execution. Next, the APC gets the per-CPU DFSS data structure pointer and acquires the idle-only queue lock referenced earlier. It then checks whether the idle-only queue on the current processor (which comes from the per-CPU quota entry contained in the process’ CPU quota block) is empty. If the list is empty, it implies that this CPU has never been inserted in the sorted block list that is contained in the per-CPU DFSS data structure (part of the PspCpuQuotaControl global array). The PspInsertQuotaBlockCpuEntry function is thus called to rectify the situation.