Good to know, thanks!
There is no mention of the non-preemption in the bpf or tc-bpf man pages
or the bcc tutorials. Is it possible to change that? I would be happy to
add a note about this if pointed in the right direction.
For example, this paragraph in the man bpf page could be edited (ALLCAPS
additions)
"eBPF programs can be attached to different events. These events can
be the arrival of network packets, tracing events, classification
events by network queueing disciplines (for eBPF programs attached
to a tc(8) classifier), and other types that may be added in the
future. A new event triggers execution of the eBPF program, which
may store information about the event in eBPF maps. Beyond storing
data, eBPF programs may call a fixed set of in-kernel helper
functions. EBPF PROGRAMS ARE NEVER PREEMPTED BY THE KERNEL. THIS
INCLUDES TAIL CALLS TO OTHER EBPF PROGRAMS."
Best regards
Pablo Alvarez
On 11/28/18 17:52, Mauricio Vasquez wrote:
>
> On 11/28/18 1:50 PM, Pablo Alvarez via Lists.Iovisor.Org wrote:
>> Dear eBPF community
>>
>> If I understand correctly, per_cpu maps are meant to avoid threading
>> issues, and for example the need to call BPF_XADD to keep counters safe.
>> There is something I am unclear about:
>>
>> - Is it possible for a BPF program to be preempted by another BPF
>> program on the same CPU?
>
> eBPF programs are never preempted by the kernel, this allows to keep
> counters in per_cpu maps without need to use synchronized primitives on
> them.
>
> This is also guaranteed that there is not preemption in a chain of tail
> calls, then per_cpu maps can also be used to store "global variables".
>
>> If this is the case, it seems that the following scenario could arise:
>>
>> BPF program 1 on cpu 0 reads a counter and is preempted before
>> incrementing it
>> BPF program 2 on cpu 0 reads the same counter, increments it, and
>> finishes
>> BPF program 1 on cpu 0 increments the counter
>>
>> At which point both programs would have read the same value for the
>> counter, with possible problems ensuing.
>>
>>
>> Is this a valid scenario? Am I missing something about how the per_cpu
>> maps are intended to be used?
>>
>> Thanks
>>
>> Pablo Alvarez
>>
>>
>>
>>
>
>
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