[PATCH net-next v3 0/3] Per epoll context busy poll support

Joe Damato
2024-01-25T22:57:45Z

Greetings:

Welcome to v3. Cover letter updated from v2 to explain why ioctl and
adjusted my cc_cmd to try to get the correct people in addition to folks
who were added in v1 & v2. Labeled as net-next because it seems networking
related to me even though it is fs code.

TL;DR This builds on commit bf3b9f6372c4 ("epoll: Add busy poll support to
epoll with socket fds.") by allowing user applications to enable
epoll-based busy polling and set a busy poll packet budget on a per epoll
context basis.

This makes epoll-based busy polling much more usable for user
applications than the current system-wide sysctl and hardcoded budget.

To allow for this, two ioctls have been added for epoll contexts for
getting and setting a new struct, struct epoll_params.

ioctl was chosen vs a new syscall after reviewing a suggestion by Willem
de Bruijn [1]. I am open to using a new syscall instead of an ioctl, but it
seemed that:
- Busy poll affects all existing epoll_wait and epoll_pwait variants in
the same way, so new verions of many syscalls might be needed. It
seems much simpler for users to use the correct
epoll_wait/epoll_pwait for their app and add a call to ioctl to enable
or disable busy poll as needed. This also probably means less work to
get an existing epoll app using busy poll.

- previously added epoll_pwait2 helped to bring epoll closer to
existing syscalls (like pselect and ppoll) and this busy poll change
reflected as a new syscall would not have the same effect.

Note: patch 1/4 uses an xor so that busy poll is only enabled if the
per-context busy poll usecs is set or the system-wide sysctl. If both are
enabled, busy polling does not happen. Calling this out specifically incase
there are strong feelings about this one; I felt one xor the other made
sense, but I am open to changing it.

Longer explanation:

Presently epoll has support for a very useful form of busy poll based on
the incoming NAPI ID (see also: SO_INCOMING_NAPI_ID [2]).

This form of busy poll allows epoll_wait to drive NAPI packet processing
which allows for a few interesting user application designs which can
reduce latency and also potentially improve L2/L3 cache hit rates by
deferring NAPI until userland has finished its work.

The documentation available on this is, IMHO, a bit confusing so please
allow me to explain how one might use this:

1. Ensure each application thread has its own epoll instance mapping
1-to-1 with NIC RX queues. An n-tuple filter would likely be used to
direct connections with specific dest ports to these queues.

2. Optionally: Setup IRQ coalescing for the NIC RX queues where busy
polling will occur. This can help avoid the userland app from being
pre-empted by a hard IRQ while userland is running. Note this means that
userland must take care to call epoll_wait and not take too long in
userland since it now drives NAPI via epoll_wait.

3. Optionally: Consider using napi_defer_hard_irqs and gro_flush_timeout to
further restrict IRQ generation from the NIC. These settings are
system-wide so their impact must be carefully weighed against the running
applications.

4. Ensure that all incoming connections added to an epoll instance
have the same NAPI ID. This can be done with a BPF filter when
SO_REUSEPORT is used or getsockopt + SO_INCOMING_NAPI_ID when a single
accept thread is used which dispatches incoming connections to threads.

5. Lastly, busy poll must be enabled via a sysctl
(/proc/sys/net/core/busy_poll).

Please see Eric Dumazet's paper about busy polling [3] and a recent
academic paper about measured performance improvements of busy polling [4]
(albeit with a modification that is not currently present in the kernel)
for additional context.

The unfortunate part about step 5 above is that this enables busy poll
system-wide which affects all user applications on the system,
including epoll-based network applications which were not intended to
be used this way or applications where increased CPU usage for lower
latency network processing is unnecessary or not desirable.

If the user wants to run one low latency epoll-based server application
with epoll-based busy poll, but would like to run the rest of the
applications on the system (which may also use epoll) without busy poll,
this system-wide sysctl presents a significant problem.

This change preserves the system-wide sysctl, but adds a mechanism (via
ioctl) to enable or disable busy poll for epoll contexts as needed by
individual applications, making epoll-based busy poll more usable. Note
that this change includes an xor allowing only the per-context busy poll or
the system wide sysctl, not both. If both are enabled, busy polling does
not happen. Calling this out specifically incase there are strong feelings
about this one; I felt one xor the other made sense, but I am open to
changing it.

Thanks,
Joe

v2 -> v3:
- cover letter updated to mention why ioctl seems (to me) like a better
choice vs a new syscall.

- patch 3/4 was modified in 3 ways:
- when an unknown ioctl is received, -ENOIOCTLCMD is returned instead
of -EINVAL as the ioctl documentation requires.
- epoll_params.busy_poll_budget can only be set to a value larger than
NAPI_POLL_WEIGHT if code is run by privileged (CAP_NET_ADMIN) users.
Otherwise, -EPERM is returned.
- busy poll specific ioctl code moved out to its own function. On
kernels without busy poll support, -EOPNOTSUPP is returned. This also
makes the kernel build robot happier without littering the code with
more #ifdefs.

- dropped patch 4/4 after Eric Dumazet's review of it when it was sent
independently to the list [5].

v1 -> v2:
- cover letter updated to make a mention of napi_defer_hard_irqs and
gro_flush_timeout as an added step 3 and to cite both Eric Dumazet's
busy polling paper and a paper from University of Waterloo for
additional context. Specifically calling out the xor in patch 1/4
incase it is missed by reviewers.

- Patch 2/4 has its commit message updated, but no functional changes.
Commit message now describes that allowing for a settable budget helps
to improve throughput and is more consistent with other busy poll
mechanisms that allow a settable budget via SO_BUSY_POLL_BUDGET.

- Patch 3/4 was modified to check if the epoll_params.busy_poll_budget
exceeds NAPI_POLL_WEIGHT. The larger value is allowed, but an error is
printed. This was done for consistency with netif_napi_add_weight,
which does the same.

- Patch 3/4 the struct epoll_params was updated to fix the type of the
data field; it was uint8_t and was changed to u8.

- Patch 4/4 added to check if SO_BUSY_POLL_BUDGET exceeds
NAPI_POLL_WEIGHT. The larger value is allowed, but an error is
printed. This was done for consistency with netif_napi_add_weight,
which does the same.

[1]: https://lore.kernel.org/lkml/65b1cb7f73a6a_250560294bd@willemb.c.googlers.com.notmuch/
[2]: https://lore.kernel.org/lkml/20170324170836.15226.87178.stgit@localhost.localdomain/
[3]: https://netdevconf.info/2.1/papers/BusyPollingNextGen.pdf
[4]: https://dl.acm.org/doi/pdf/10.1145/3626780
[5]: https://lore.kernel.org/lkml/CANn89i+uXsdSVFiQT9fDfGw+h_5QOcuHwPdWi9J=5U6oLXkQTA@mail.gmail.com/

Joe Damato (3):
eventpoll: support busy poll per epoll instance
eventpoll: Add per-epoll busy poll packet budget
eventpoll: Add epoll ioctl for epoll_params

.../userspace-api/ioctl/ioctl-number.rst | 1 +
fs/eventpoll.c | 122 +++++++++++++++++-
include/uapi/linux/eventpoll.h | 12 ++
3 files changed, 130 insertions(+), 5 deletions(-)

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2.25.1