1======================================= 2The padata parallel execution mechanism 3======================================= 4 5:Last updated: for 2.6.36 6 7Padata is a mechanism by which the kernel can farm work out to be done in 8parallel on multiple CPUs while retaining the ordering of tasks. It was 9developed for use with the IPsec code, which needs to be able to perform 10encryption and decryption on large numbers of packets without reordering 11those packets. The crypto developers made a point of writing padata in a 12sufficiently general fashion that it could be put to other uses as well. 13 14The first step in using padata is to set up a padata_instance structure for 15overall control of how tasks are to be run:: 16 17 #include <linux/padata.h> 18 19 struct padata_instance *padata_alloc(const char *name, 20 const struct cpumask *pcpumask, 21 const struct cpumask *cbcpumask); 22 23'name' simply identifies the instance. 24 25The pcpumask describes which processors will be used to execute work 26submitted to this instance in parallel. The cbcpumask defines which 27processors are allowed to be used as the serialization callback processor. 28The workqueue wq is where the work will actually be done; it should be 29a multithreaded queue, naturally. 30 31To allocate a padata instance with the cpu_possible_mask for both 32cpumasks this helper function can be used:: 33 34 struct padata_instance *padata_alloc_possible(struct workqueue_struct *wq); 35 36Note: Padata maintains two kinds of cpumasks internally. The user supplied 37cpumasks, submitted by padata_alloc/padata_alloc_possible and the 'usable' 38cpumasks. The usable cpumasks are always a subset of active CPUs in the 39user supplied cpumasks; these are the cpumasks padata actually uses. So 40it is legal to supply a cpumask to padata that contains offline CPUs. 41Once an offline CPU in the user supplied cpumask comes online, padata 42is going to use it. 43 44There are functions for enabling and disabling the instance:: 45 46 int padata_start(struct padata_instance *pinst); 47 void padata_stop(struct padata_instance *pinst); 48 49These functions are setting or clearing the "PADATA_INIT" flag; 50if that flag is not set, other functions will refuse to work. 51padata_start returns zero on success (flag set) or -EINVAL if the 52padata cpumask contains no active CPU (flag not set). 53padata_stop clears the flag and blocks until the padata instance 54is unused. 55 56The list of CPUs to be used can be adjusted with these functions:: 57 58 int padata_set_cpumasks(struct padata_instance *pinst, 59 cpumask_var_t pcpumask, 60 cpumask_var_t cbcpumask); 61 int padata_set_cpumask(struct padata_instance *pinst, int cpumask_type, 62 cpumask_var_t cpumask); 63 int padata_add_cpu(struct padata_instance *pinst, int cpu, int mask); 64 int padata_remove_cpu(struct padata_instance *pinst, int cpu, int mask); 65 66Changing the CPU masks are expensive operations, though, so it should not be 67done with great frequency. 68 69It's possible to change both cpumasks of a padata instance with 70padata_set_cpumasks by specifying the cpumasks for parallel execution (pcpumask) 71and for the serial callback function (cbcpumask). padata_set_cpumask is used to 72change just one of the cpumasks. Here cpumask_type is one of PADATA_CPU_SERIAL, 73PADATA_CPU_PARALLEL and cpumask specifies the new cpumask to use. 74To simply add or remove one CPU from a certain cpumask the functions 75padata_add_cpu/padata_remove_cpu are used. cpu specifies the CPU to add or 76remove and mask is one of PADATA_CPU_SERIAL, PADATA_CPU_PARALLEL. 77 78If a user is interested in padata cpumask changes, he can register to 79the padata cpumask change notifier:: 80 81 int padata_register_cpumask_notifier(struct padata_instance *pinst, 82 struct notifier_block *nblock); 83 84To unregister from that notifier:: 85 86 int padata_unregister_cpumask_notifier(struct padata_instance *pinst, 87 struct notifier_block *nblock); 88 89The padata cpumask change notifier notifies about changes of the usable 90cpumasks, i.e. the subset of active CPUs in the user supplied cpumask. 91 92Padata calls the notifier chain with:: 93 94 blocking_notifier_call_chain(&pinst->cpumask_change_notifier, 95 notification_mask, 96 &pd_new->cpumask); 97 98Here cpumask_change_notifier is registered notifier, notification_mask 99is one of PADATA_CPU_SERIAL, PADATA_CPU_PARALLEL and cpumask is a pointer 100to a struct padata_cpumask that contains the new cpumask information. 101 102Actually submitting work to the padata instance requires the creation of a 103padata_priv structure:: 104 105 struct padata_priv { 106 /* Other stuff here... */ 107 void (*parallel)(struct padata_priv *padata); 108 void (*serial)(struct padata_priv *padata); 109 }; 110 111This structure will almost certainly be embedded within some larger 112structure specific to the work to be done. Most of its fields are private to 113padata, but the structure should be zeroed at initialisation time, and the 114parallel() and serial() functions should be provided. Those functions will 115be called in the process of getting the work done as we will see 116momentarily. 117 118The submission of work is done with:: 119 120 int padata_do_parallel(struct padata_instance *pinst, 121 struct padata_priv *padata, int cb_cpu); 122 123The pinst and padata structures must be set up as described above; cb_cpu 124specifies which CPU will be used for the final callback when the work is 125done; it must be in the current instance's CPU mask. The return value from 126padata_do_parallel() is zero on success, indicating that the work is in 127progress. -EBUSY means that somebody, somewhere else is messing with the 128instance's CPU mask, while -EINVAL is a complaint about cb_cpu not being 129in that CPU mask or about a not running instance. 130 131Each task submitted to padata_do_parallel() will, in turn, be passed to 132exactly one call to the above-mentioned parallel() function, on one CPU, so 133true parallelism is achieved by submitting multiple tasks. parallel() runs with 134software interrupts disabled and thus cannot sleep. The parallel() 135function gets the padata_priv structure pointer as its lone parameter; 136information about the actual work to be done is probably obtained by using 137container_of() to find the enclosing structure. 138 139Note that parallel() has no return value; the padata subsystem assumes that 140parallel() will take responsibility for the task from this point. The work 141need not be completed during this call, but, if parallel() leaves work 142outstanding, it should be prepared to be called again with a new job before 143the previous one completes. When a task does complete, parallel() (or 144whatever function actually finishes the job) should inform padata of the 145fact with a call to:: 146 147 void padata_do_serial(struct padata_priv *padata); 148 149At some point in the future, padata_do_serial() will trigger a call to the 150serial() function in the padata_priv structure. That call will happen on 151the CPU requested in the initial call to padata_do_parallel(); it, too, is 152run with local software interrupts disabled. 153Note that this call may be deferred for a while since the padata code takes 154pains to ensure that tasks are completed in the order in which they were 155submitted. 156 157The one remaining function in the padata API should be called to clean up 158when a padata instance is no longer needed:: 159 160 void padata_free(struct padata_instance *pinst); 161 162This function will busy-wait while any remaining tasks are completed, so it 163might be best not to call it while there is work outstanding. 164