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diff --git a/Documentation/padata.txt b/Documentation/padata.txt new file mode 100644 index 000000000000..269d7d0d8335 --- /dev/null +++ b/Documentation/padata.txt @@ -0,0 +1,107 @@ +The padata parallel execution mechanism +Last updated for 2.6.34 + +Padata is a mechanism by which the kernel can farm work out to be done in +parallel on multiple CPUs while retaining the ordering of tasks. It was +developed for use with the IPsec code, which needs to be able to perform +encryption and decryption on large numbers of packets without reordering +those packets. The crypto developers made a point of writing padata in a +sufficiently general fashion that it could be put to other uses as well. + +The first step in using padata is to set up a padata_instance structure for +overall control of how tasks are to be run: + + #include <linux/padata.h> + + struct padata_instance *padata_alloc(const struct cpumask *cpumask, + struct workqueue_struct *wq); + +The cpumask describes which processors will be used to execute work +submitted to this instance. The workqueue wq is where the work will +actually be done; it should be a multithreaded queue, naturally. + +There are functions for enabling and disabling the instance: + + void padata_start(struct padata_instance *pinst); + void padata_stop(struct padata_instance *pinst); + +These functions literally do nothing beyond setting or clearing the +"padata_start() was called" flag; if that flag is not set, other functions +will refuse to work. + +The list of CPUs to be used can be adjusted with these functions: + + int padata_set_cpumask(struct padata_instance *pinst, + cpumask_var_t cpumask); + int padata_add_cpu(struct padata_instance *pinst, int cpu); + int padata_remove_cpu(struct padata_instance *pinst, int cpu); + +Changing the CPU mask has the look of an expensive operation, though, so it +probably should not be done with great frequency. + +Actually submitting work to the padata instance requires the creation of a +padata_priv structure: + + struct padata_priv { + /* Other stuff here... */ + void (*parallel)(struct padata_priv *padata); + void (*serial)(struct padata_priv *padata); + }; + +This structure will almost certainly be embedded within some larger +structure specific to the work to be done. Most its fields are private to +padata, but the structure should be zeroed at initialization time, and the +parallel() and serial() functions should be provided. Those functions will +be called in the process of getting the work done as we will see +momentarily. + +The submission of work is done with: + + int padata_do_parallel(struct padata_instance *pinst, + struct padata_priv *padata, int cb_cpu); + +The pinst and padata structures must be set up as described above; cb_cpu +specifies which CPU will be used for the final callback when the work is +done; it must be in the current instance's CPU mask. The return value from +padata_do_parallel() is a little strange; zero is an error return +indicating that the caller forgot the padata_start() formalities. -EBUSY +means that somebody, somewhere else is messing with the instance's CPU +mask, while -EINVAL is a complaint about cb_cpu not being in that CPU mask. +If all goes well, this function will return -EINPROGRESS, indicating that +the work is in progress. + +Each task submitted to padata_do_parallel() will, in turn, be passed to +exactly one call to the above-mentioned parallel() function, on one CPU, so +true parallelism is achieved by submitting multiple tasks. Despite the +fact that the workqueue is used to make these calls, parallel() is run with +software interrupts disabled and thus cannot sleep. The parallel() +function gets the padata_priv structure pointer as its lone parameter; +information about the actual work to be done is probably obtained by using +container_of() to find the enclosing structure. + +Note that parallel() has no return value; the padata subsystem assumes that +parallel() will take responsibility for the task from this point. The work +need not be completed during this call, but, if parallel() leaves work +outstanding, it should be prepared to be called again with a new job before +the previous one completes. When a task does complete, parallel() (or +whatever function actually finishes the job) should inform padata of the +fact with a call to: + + void padata_do_serial(struct padata_priv *padata); + +At some point in the future, padata_do_serial() will trigger a call to the +serial() function in the padata_priv structure. That call will happen on +the CPU requested in the initial call to padata_do_parallel(); it, too, is +done through the workqueue, but with local software interrupts disabled. +Note that this call may be deferred for a while since the padata code takes +pains to ensure that tasks are completed in the order in which they were +submitted. + +The one remaining function in the padata API should be called to clean up +when a padata instance is no longer needed: + + void padata_free(struct padata_instance *pinst); + +This function will busy-wait while any remaining tasks are completed, so it +might be best not to call it while there is work outstanding. Shutting +down the workqueue, if necessary, should be done separately. |