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    SubjectRe: [PATCH] Add a document for the padata interface
    On Fri, 14 May 2010 12:58:59 +0200
    Steffen Klassert <> wrote:

    > We disable the BHs before we call parallel(), so sleeping is not allowed.
    > This is necessary to keep up with the networking softirq if padata is used
    > for doing IPsec. didn't point that out last time :)

    I've updated that one sentence in the attached patch; everything else
    is the same. Look good?



    Add a document describing the padata interface

    This originally appeared as

    Cc: Steffen Klassert <>
    Signed-off-by: Jonathan Corbet <>
    Documentation/00-INDEX | 2 +
    Documentation/padata.txt | 107 ++++++++++++++++++++++++++++++++++++++++++++++
    2 files changed, 109 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/padata.txt

    diff --git a/Documentation/00-INDEX b/Documentation/00-INDEX
    index 06b982a..dd10b51 100644
    --- a/Documentation/00-INDEX
    +++ b/Documentation/00-INDEX
    @@ -250,6 +250,8 @@ numastat.txt
    - info on how to read Numa policy hit/miss statistics in sysfs.
    - how to decode those nasty internal kernel error dump messages.
    + - An introduction to the "padata" parallel execution API
    - directory with info on using Linux on PA-RISC architecture.
    diff --git a/Documentation/padata.txt b/Documentation/padata.txt
    new file mode 100644
    index 0000000..269d7d0
    --- /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
    +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
    +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.

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